Austentite Steel Thermal Stabilization

1149

SPECIAL

The Second John Player Lecture

STEELS FOR HIGH TEMPERATURE SERVICE

By H. W. Kirkby*

In this lecture the author reports on the progress made in the development and application of creep-resisting steels over the last 20 years, with particular reference to research carried out at The Brown-Firth Research Laboratories. The properties of the more established steels are reviewed, along with those of a number of newer steels introduced during this period. These include the 12 per cent chromium variants specially ‘tailor- made’ for aircraft gas turbine discs, two improved austenitic steels and two non-stainless types: a 6 per cent chromium complex steel, and a 2 per cent chromium molybdenum vanadium aluminium nitriding type. Cast austenitic pipe steels are also discussed.

The author describes work being carried out with the object of improving the proof strength of austenitic steels, e.g. warm working and alloying with nitrogen.

Some information is also presented in respect of the creep ductility properties of a number of the steels mentioned in the paper, with special reference to weld heat effects.

INTRODUCTION THE LAST 20 YEARS have seen a remarkable expansion of activity in the field of high temperature metallurgy and a large growth in the tonnage and variety of special steels being put into service at elevated temperatures. This growth of activity can be directly related to the rapid advances which have been made in various specialized engineering fields, such as those concerned with jet engines and other forms of gas turbine, steam turbines, boilers, etc. and nuclear power. A more recent user of high temperature steels is the gas industry. This is due to the revolu- tionary changes which are taking place in gas-making since the adoption of the I.C.I. reformer process.

A result of this activity has been the development of a number of new steels, some of which have been ‘tailor- made’ to match a particular set of conditions. Other steels have been evolved with the end point less specific though with a general direction in mind. Some steels have been improved by trimming existing compositions after manufacturing and service experience but only occasionally does a steel turn up ‘out of the blue’ with properties entirely unexpected. By and large the development of special steels follows an evolutionary pattern needing a technological approach with the sort of stimulus usually provided

The MS. of this lecture was received at the Institution an 5thJanuary

* Director of Research, Firth Bromn Limited. Proc Instn Mech Engrs 1965-66

1966.

by engineers. It is a matter of history that hardly any steels have been developed in an ‘academic vacuum’.

The past two decades have also seen a tremendous growth in the practice of joining by welding which has undoubtedly complicated the task of the metallurgist. As the properties of steels are determined largely by a com- bination of composition and heat treatment, and welding introduces almost every conceivable form of heat treatment, it is expecting a lot of the metallurgist always to provide acceptable properties when steels are subjected to such conditions. Reference to weld heat effects is made in various parts of the lecture.

Apart from the question of new or modified steels, a considerable amount of data is being collected on the so- called standard steels which has only been made possible by a large increase in testing facilities throughout industry. 10000 h creep tests are commonplace today and many programmes cater for test times well in excess of this with targets of 50 000 to 100000 h. Short time creep rate methods or extrapolations from, say, 1000 h tests are no longer viable for the plant lives and temperatures now being considered.

The extension of testing facilities has also been necessary in order to determine scatter bands for steels of nominally the same composition and heat treatment and covering creep, rupture and proof strength over a wide range of temperatures. Mean and minimum data curves are being

VoI I80 ft I

1150 1%. W. KIRKBY

Mn 1 Cr ________

0.50 0.90

0.45 2.25

0.50 0.50 0.50 9.30

determined for design purposes. This information is also contributing to the establishment of boiler code working stresses, e.g. I.S.O. and B.S. 1515.

In this lecture the author presents information obtained on a number of comparatively new steels developed at The Brown-Firth Research Laboratories along with up- to-date high temperature data on ‘standard’ steels or slightly modified steels. Where possible the presentation of the properties has been related to criteria such as 0-1-0.2 per cent creep deformation or rupture in 100 000 h or, for the sub-creep temperatures, the proof strength.

The author also endeavours to focus attention on the subject of creep ductility with special reference to the influence of welding on this property.

An important section of the lecture describes some of the work being carried out to improve the proof strength of austenitic steels. A new role for cast austenitic steels is also described.

Mo

0.55

1.00

0.60 1.05

N O N - S T A I N L E S S ALLOY STE-ELS Most of the steels to be discussed in this section are essentially those used for pipes, tubes, headers, etc., for applications at elevated temperatures up to 1050°F (565°C). The compositions, heat treatments, etc., are detailed in Tables 1, 2, and 3.

The steels listed in Table 1 include the now ‘standard’

- -

0.25 -

1% CrMo, 2$y0 Cr 1% Mo and 3% CrMoV materials whilst Tables 2 and 3 give details of two new compositions, namely (1) Rex 500, a complex 6% Cr type, and (2) HK5, a new nitriding creep-resisting steel.

As the creep design criterion for these steels when used for pipes etc. is usually some fraction (60-66 per cent) of the rupture strength in 100 000 h, Fig. 1 has been com- piled from average rupture strength estimates for these steels at a number of temperatures. In most instances the estimates have been obtained from test results extending beyond 30 000 h and cover several different casts of steel for each composition.

Both the 2$y0 Cr 1% Mo and 3% CrMoV steels have been in service in British power stations at 1000°F (538°C) and 1050°F (565°C) for a number of years with no reported failures, but the former steel has now been superseded by the 4% CrMoV steel largely for cost reasons. The two steels have equivalent rupture strengths but comparison of the respective ductility behaviour under creep conditions shows that the 2;t% Cr 1% Mo steel is more ductile than the 5% CrMoV steel. This is illustrated by the following minimum creep ductility values obtained on testpieces stressed in the temperature range 525- 575°C:

Normalize 930-950°C Stress relieve or temper at 650°C maximum Anneal 930-950°C or Normalize and temper 650-750°C Normalize 950-975°C and temper 670-700°C Anneal 920-930°C or

+% CrMoV

0-12

0.10 0.11

6-7 per cent elongation

0.25

0.25 0.40

2+% Cr 1% Mo 10-12 per cent elongation

2t7; CrMo +% CrMoV -- ~ 1% CrMo

Table 1. Analysis, heat treatment and tensile properties of pipe steels: 1% CrMo, 2*% CrMo, 3% CrMoV and 9% CrMo

9% CrMo

1% CrMo . 24% CrMo.

+% CrMoV 9% CrMo .

c j s i

20°C 200°C 400°C 600°C

i 20°C ~

600°C

21.4 21.6 31.4 42 74 19.3 19.4 29.9 37 67 14.1 15.8 30.7 33 65 11.6 13.2 2081 42 87

Annealed 940°C (cooled at average rate of 60 degC/h

between 940°C and 700°C)

Annealed

18.2 18.2 34.2 33 64 22.5 22.5 32.1 29 78 15.4 15.5 28.4 31 62 20.4 20.4 30.9 23 78 12.4 13.7 31.7 25.5 55 ’ 14.0 15.4 29.5 23 75 11.1 12.1 21.8 42 79.5, 11.6 12.0 18.3 35 89

Normalized at 950°C and 1 tempered at 680°C

39.3 394 47.1 29 77 35.8 36.7 42.9 23.5 76 33.6 34.8 41.0 25 74.5 22.0 23.8 27.2 31 86 I

16.1 16.3 34.3 38 66 12.4 12.7 28.7 36 66 10.8 11.9 26.5 30 61 8.3 9.2 16.4 48 81

Normalized and tempered

35.0 36.2 463 30 71 32.4 33.3 40.8 26 70 29-4 30.7 37.8 21 64 19.1 21.4 23-6 35 87

Vo1180 Pi I Proc Instra Mech Engrs 1965-66

SPECIAL STEELS FOR HIGH TEMPERATURE SERVICE 1151

I I 500 550 600 650 700

TEMPERATURE - ‘C

Fig. I . 100 000 h rupture strength

The 6-7 per cent ductility of the 4% CrMoV steel is probably more than adequate for successful service as the approximate average deformation using a 60-66 per cent rupture strength criterion would only be of the order of 0.2-0.3 per cent in 100 000 h. On a similar basis, the 2$% Cr 1% Mo steel would have a deformation of about 0.5 per cent. A difficulty arises, however, in assessing the properties in the vicinity of welds since many steels show a lower creep ductility habit (and a higher creep resistance) when subjected to the very high solution heat treatments which occur during welding. Results from simulated tests using solution temperatures and times approximating to those reached at the weld-parent metal junction have shown that the creep ductility elongation value of the -;yo CrMoV steel can fall to quite a low level, elongation figures of 1 per cent and lower being obtained at test temperatures of 525-575°C. The 24% Cr 1% Mo steel, on the other hand, exhibits 5 per cent or more ductility when tested under the same conditions.

The 4% CrMoV components now in service were welded with weld metal of the 247; Cr 1% Mo composition following initial tests with 37; CrMoV weld metal. This latter had unsatisfactory properties. Various investigators (I) (2)* have studied the creep-rupture behaviour of the 3% CrMoV welded with 2+y0 Cr 1% Mo and the results show that whilst the composite weld-parent metal testpieces fracture initially in the 2$y0 Cr 1% Mo weld metal, those tested for longer times show an increasing tendency to fail at the weld junction, i.e. the zone of highest solution treatment.

Although a low ductility habit at a weld junction does not necessarily imply premature failure in service, it may constitute an added risk if bending-longitudinal stresses are in excess of those allowed for in the basic design.

There are other variations of the basic MoV composition being used in steam and gas turbines. For example, a +-1% CrMoV type is employed in cast form for turbine casings, steam chests, etc., and these also are welded with 2&Y0 Cr 1% Mo weld metal. Another type is the 0.25°/0 C lyO CrMoV steel which has now been virtually standard- ized for high-pressure and intermediate pressure rotors. * References are given in the Appendix. Proc Instn Mech Rtrgrs 196566

Welding is not usually carried out in these latter cases. Some large forged steam chests are also made to the pipe steel composition and these, of course, are subjected to welding. The points made earlier in regard to the effects of welding apply in the cases just mentioned.

Included in Fig. 1 is a plot for the 9% CrMo steel which has rupture properties similar to those of the 3% CrMoV and 24o;b Cr ly0 Mo steels. The 9% CrMo steel has been used extensively in the oil industry for many years but it is now to be used in steam power plant work for reheater piping to operate at 1050°F (565°C) on account of its superior scaling resistance. It has extremely good creep ductility, minimum values of the order of 15-20 per cent elongation being obtained with the normally treated wrought product. Simulated welding treatments carried out on 9% CrMo testpieces indicate minimum values of 7-8 per cent elongation and in this respect it is much superior to both 37; CrMoV and 2$y0 Cr 1% Mo steels.

Complex 6 per cent chromium steel (Rex 500) C Cr Mo W V Ti B

0.08 6.0 1.0 0.50 0.20 0.25 0,006 per cent The development of this steel followed an initial study

some years ago to determine whether a material, having the creep properties of +yo CrMoV steel, could be devised with a chromium content of 6 per cent in order to provide some 50 degC improvement in scaling resistance over 3% CrMoV steel. Straight additions of chromium in excess of 1 per cent were known to have a deleterious effect on creep resistance of the basic MoV composition but it was found that the resistance could be maintained with up to 3% Cr if an addition of vanadium was made equivalent to 4 x carbon content. Above this level of chromium, however, creep properties again fell away rapidly with a minimum around 6-7;/, Cr.

By adding molybdenum, tungsten, vanadium, titanium and boron in suitable proportions, a composition was discovered which turned out to have a rupture strength at 600°C as high as that of the +yo CrMoV at 550°C. From Fig. 1 it will be noted that Rex 500 is superior to the 18/12/1 Nb austenitic steel for temperatures up to 575°C with an equivalent strength at 600°C.

The general physical and mechanical properties of this material are summarized in Table 2, from which it will be seen that Rex 500 is a 55-60 tons tensile steel with a relatively high transition temperature, i.e. low impact values at ambient temperature. Above 150°C impact properties are high.

This steel is to be used for turbine inlet steam piping in some of the newer 1050°F 500-MW sets. It offers possibilities as a superheater tube material for temperatures up to 1100°F (593°C) where austenitic steels are currently being employed.

The composition balance of this material is extremely critical particularly in regard to the carbon/titanium ratio, necessitating very accurate melting and testing control. At the moment the material is being welded using

Vol180 Pr I

1152 H. W. KIRKBY

Table 2. 6% CrMoVWTi steel-Rex 500

Rex 500 is a weldable 69/0 chromium martensitic steel with creep rupture strength superior to the conventional molybdenum vanadium types. It can be readily produced as hot or cold finished pipes or tubes and as bar or small forgings. Typical composition:

C Cr Mo W V T i B 0.08 6.00 1.0 0.50 0.20 0.25 0.005

Heat treatment: A.C.1150-1170°C. Temper 650-7OO'C.

Coefficient of thermal expansion (in/in/degC) : 20-100"c 0.000 010 20-300°C 0.000 01 1 20-500°C 0.000 012 20-700°C 0.000 013

Tensile tests at elevated temperatures (bar material-A.C.1150"C. Temper 700°C):

- .~ ~~

- 20°C

100°C 200°C

300°C 400°C 500°C 550°C 600°C 650°C __

I

L.P. P.S. P.S. P.S. P.S. 1 0-050/, 0.1% 0.2% 0.5% U.T.S. E R.A. Izod value, yo ~ yo ~ f t lb

33.0 41.5 43.2 44.7 46.0 55.0 I225 64 1 3 32.9 40.3 42.0 43-0 43.8 48.7 123 1 6 6 5 4 32.5 39.1 40.7 41.8 42-2 46.3 122.5 67.5 78

1 1 I 120at 150°C 29.0 36.6 38.6 40.0 41.3 44.5 ' 2 0 1 65.5 I - 26.0 35.0 36.6 38.2 39.3 42-8 119 1 65.5 I - 24.0 32.4 34.1 35.6 37.6 39.9 20 65.5 1 - 18.0 28.6 30.8 32.7 34.0 36.0 121 71 - 16.0 24.8 26.4 28.0 29.4 32.2 124 i 74.5 I - 10.0 17.6 20.2 22.2 24.2 26.8 28 78.5 ~ - I

Rupture strength: Stress in ton/inZ to give rupture in times specified _ ~ _ ~

1 566°C ! 600°C ' 650°C 1 (1050°F) , (1112°F) j (1202°F)

1 0 0 0 h 1 23 ~ 19 ~ 8.5

( ) denotes extrapolated values. All stresses given above are mean values derived from scatter

band data on bar and pipes. '

Inconel 182 weld metal* but work is in progress with the object of developing an electrode much nearer to the parent composition. Creep ductility studies are in the process of being carried out similar to those mentioned in connection with the other steels.

2% CrMoVAl (HK5) steel C Cr Mo V A1

0.25 2.0 1.0 0.45 0.6 per cent HK5 was 'tailor-made' to meet a request for a superior

creep-resisting nitriding steel suitable for use up to 1050°F (565°C) for parts where resistance to seizing is important. This steel has been in service for about 3 years * C Mn Ni Cr Mo Nb Ti

Inconel 182 0.03 8.0 64 14 0.4 1.8 0.3 per cent Proc Instn Mech Engrs 196.7-66

in the form of emergency stop valve stems and bushes in 950-1000°F (510-538°C) turbines. It is to be used for similar duty in 1050°F (565°C) turbines.

HK5 is a development of the 1% CrMoAl steel (En41) which hitherto has been used for similar valve parts at temperatures up to 900-950°F (482-510°C) on account of its excellent resistance to seizing. The 17; CrMoAl steel is easy to nitride and has a high nitrided case hardness with an excellent resistance to softening with time at temperature. The problem was to combine these properties with a higher creep resistance as steam temperatures were increased.

As a first step, the behaviour of a number of low alloy creep-resisting steels was examined by nitriding at 500°C and exposing to a temperature of 550°C. Steels of the 3% CrMo, 3% CrMoWV and lo/, CrMoV type were found to nitride satisfactorily but none had the necessary resistance to softening. The l2;4 Cr creep-resisting steels were also tested in this way but in all instances brittle nitrided cases with poor resistance to softening were obtained. It was eventually found that an aluminium addition of 0-5 per cent or more was a basic essential for achieving high case hardness with good resistance to softening. The remainder of the composition was balanced to give the required creep resistance and a hardenability suitable for the forgings to be made.

The results of room temperature hardness tests after exposure at 550°C are plotted in Fig. 2 for the HK5 and 1 CrMoAl steels, comparison curves being provided for the 1% CrMoV, 3q4, CrMo and 324 CrMoWV steels. Fig. 2 also shows hardness curves after exposure at 600°C for the HK5 and 17; CrMoAl steels. The superiority of the aluminium-bearing steels is evident.

It should be borne in mind that in all cases the results plotted in Fig. 2 were produced by nitriding at 500°C which is standard for general purpose nitriding parts. This largely explains the initial drop in hardness on exposure at 550 and 600°C.

100

3 %Cr Ma W V 550°C HCM 5 550'C

( 3 % C r Mo)

2 C 0

EXPOSURE TIME -months

Fig. 2. Eflect of exposure at 550°C and 600°C on maximum case hardness at 2OoC-2% CrMoVAl (HK5) steel

Vol180 PL 1


E %

26 24 23 21 21

2or

R.A. Yo

-- 67 67.5 67 63 60

10 -0 Id000 .I

DURATION - hours

Fig. 3. Comparison of creep behaviour-550°C

Using a 0.1 per cent creep deformation limit at 550"C, a comparison of the creep behaviour of HK5,1% CrMoV (forging type), 3% CrMo (En 40) and 1% CrMoAl steels is made in Fig. 3 which shows that the HK5 has equivalent resistance to that of the l y i CrMoV forging steel. Rupture strength behaviour was also found to be similar for the two steels. Table 3 lists typical mechanical properties obtained on the new steel.

An interesting feature of the investigation carried out in developing HK5 steel was that large amounts of aluminium within the limits 0 to 1.5 per cent did not have any noticeable effect on creep behaviour provided that vanadium was present. Similar studies on carbon- and molybdenum-bearing steels have shown that aluminium can have a deleterious effect on creep resistance.

Table 3. 2% CrMo VAl nitriding creep-resisting steel- H K 5

C Cr Mo V A1 0-25 2.0 1.0 0.45 0.6 per cent

Heat treatment: A.C.975"C. Temper 690-710°C. I

I ton/in2

R.T. I 48.6 49.0 49.6 50.3 57-6 100°C 45.6 46.1 46.8 47.6 54-0 200°C 1 42-3 43.2 44.2 45.5 51.4 3 0 0 ~ i 41.4 42.9 4S.i 46.6 l0. i 400°C 38.6 40.4 42.1 43.9 48.0 500°C I 35.5 375 38.8 39.7 43.6 550°C ' 33.0 34.3 36.0 36.6 40.7 600°C ' 26.6 28-0 293 29.8 34.1

Creep and rupture: 550°C

Izod, ft Ib

__ 22

105 95 - - - - - __

Rupture in 1 000 h 19 ton/in2 0.1% in 1 000 h 6 ton/in2 0.1 % in 10 000 h 3.5 ton/inz

0.1% in 1 000 h 8 ton/in2 0.2% in 1000 h 8 ton/inZ 0.1 % in 10 000 h 5 ton/ina 0.2% in 10 000 h 5 ton/ina 0.1% in 30 000 h 4 ton/ina 0.2% in 30 000 h 3.5 ton/ina 0.2% in 10 000 h 7 ton/ina

10 000 h 14 ton/in2

Proc Mech Inscn Engrs 1965-66

12 PER CENT CHROMIUM MARTENSITIC STAINLESS STEELS*

(1) 12y0 CrMoVNb and 12% CrMoVNbCo steels Two 12 per cent chromium steels have been used for many years for turbine blading and other high temperature parts :

C Cr Ni Mo Plain 12% Cr (F.1.) B.S. 3s-61, En 56A 0.10 125 0.25 - per cent

12% CrMo (F.I.Mo.) 0.12 12.0 0.40 0.70 per cent It was the introduction of the jet engine, however,

which provided the real impetus for the further and more intensive development of the 12 per cent chromium stainless steel for high temperature applications.

Because of the emphasis placed on creep resistance, the turbine discs of the early aircraft gas turbines were produced from complex austenitic steels, the most successful material being Jessop's G. 18Bt. This steel as originally used had a room temperature 0.2 per cent proof strength of about 17 ton/in2 which was eventually raised to approximately 28 ton/in2 by warm working, thereby reducing considerably the deformation experienced on the low proof strength discs. As austenitic materials combining good creep strength with higher proof strength were not available at the time, designers were attracted by the possibilities of using heat-treatable martensitic steels with high proof stresses provided some means could be found to deal with the lower creep properties of this class of steel. This problem was resolved by the simple expedient of air cooling the rim of the disc to a temperature suitable for the particular material chosen.

Using this design concept, the early 'martensitic steel' engines employed both low alloy creep-resisting steels such as MoV and 3% CrMoWV and the plain 1276 Cr 0.2% C stainless steel of the S.62 type. As the much lighter discs arising from using martensitic steels increased the corrosion hazard, this led to a demand to develop further the stainless martensitic steel rather than attempt to develop improved grades of low alloy non-stainless steels.

Two steels having higher proof strengths and creep properties were eventually produced, the first being F.V.448 followed by F.V.535 which was developed a few years later to meet even more stringent demands:

C Cr Ni Mo V Nb Co Na F.V.448 F.V.535 (12% Cr 3862 for

0.11 10.7 0.80 0.65 0.18 0-35 - 0.05 per cent 0.09 10-7 0.40 0.75 0.18 0.35 6.0 - per cent

comparison) 0.23 13.60.23 - - - - - per cent Table 4 summarizes the physical and mechanical

properties of these steels along with those of the plain 12% Cr and the complex austenitic G.18B steel. I t will be noted that both the new steels have somewhat higher strengths than the plain 12% Cr steel and, of course, the austenitic material. Of special interest are the low room

* N.B. Although chromium contents in this class of steel may range p m , say, 10.5 to 13.5per cent, they are now generally designated

I 2 per cent chromium steels'. C Ni Cr Mo W Co Nb

G.18B 0.4 13.0 13.0 2.5 2.5 10.0 3.0 per cent

Vol180 Pt I

t

1154 H. W. KIRKBY

Table 4. 12% Cr martensitic stainless steels-analysis, heat treatment, physical and tensile properties

Ni

0.25 0.75 0.30

____

13.0

Heat treatment I

N2 c o

- 1 - I O.H.960"C,T.600"C

- ! l::o" I Solution treated and warm worked - 1 i

O.H. or A.C.l160'C, T.670"C O.H. or A.C.116O0C, T.650"C

0.05 - 12% Cr ' 0.23 F.V.448 j 0.10

I F.V.535 1 0.07 G.18B 0.40

13.0 11.0 10.5 13.0

Physical properties:

I 12% Cr -

F.V.535 I F.V.448 ~

G.18B

8.1 __ _____

Specific gravity 7.75, equivalent to 483 lb/ft3

7.76, equivalent to 483.5 Ib/ft3

65 microhm-cm

7-79, equivalent to 485 lb/ft3

67 microhm-cm Electrical resistivity

at 20°C at 600°C

57 microhm-cm 106 microhm-cm

93.8 microhm-cm

Coefficient of thermal

0.000 010

0~000 011 I O~OOO 010

0.000 01 1

expansion 20/100"C 20/200"c

0.000 011

0.000 01 1

0.000 012

0.000 012

0.000 015 0-000 016

0.000 017

0.000 017 0.000 012 0.000 012

I 0.000 012

I 0~000012 20)600"C 20/700"C

Tensile properties :

K.A. O f 10

Izod, ft lb

Young's modulus, ton/inz

E %

0.05% 0.1% 0.274 0.5% P.S. P.S. P.S. P.S. M.S. L.P.

ton/ina

34.0 44.7 46.5 48.2 50.3 58.6 25.0 40.2 42.8 45.7 47.8 53.5 21.0 32.9 35.7 38.7 41.5 46.9 7.5 12-7 14.2 15.7 18.6 25.1

23 22 20 40

20 20 20 19 17 21 32 29 28

22 20 19 17 18 21 22

25 20 18 22 20

61 62 59 86

56 58 56 55 55 58 74 77 77

64 67 59 52 56 68 70

31 20 25 20 22

12% Cr R.T. 200°C 400°C 600°C

F.V.448 R.T. 100°C 200°C 300°C 400°C 500°C 600°C 650°C 700°C

13 900 13 100 12 000 10 600

14 000 13 600 13 150 12 600 12 000 11 300 10 400 9 800 9 200

14 110 13 820 13 350 12 840 12 330 11 700 11 300

40-0 53.2 55.3 57.0 59.2 67.0 40.0 52.0 53.8 55.2 56.0 64.0 34.0 48-7 50.5 52.9 55.0 60.0 33.5 46.4 48.7 50-2 52-3 57.0 30.0 43.6 46.1 48.5 51.2 56.0 26.6 38-5 41.0 43.2 45.7 48.4 15.0 24-8 27.9 31-6 35.0 36.3 5 .O 16-2 21.0 25-0 28.3 29-2 3.7 13.4 16.6 19.6 21.8 23.0

F.V.535 K.T. 28.0 54.8 59.6 63.7 68.8 72.3

20.0 49.3 54.7 60.0 65.6 70-2 16.8 50.7 55.0 58.8 63.0 67.2

100°C 200°C 300T 400°C 500°C 550°C

G.18B R.T. 200°C 400°C 600°C 700°C

13.8 45.5 50.7 56.5 61.3 65.7 18.4 43.5 48.7 53.6 58.8 62.7 18.4 39.8 44.0 48.0 52.8 56.7 12.0 36.3 41.3 46.1 49.9 53.0

31.0 32.0 48.0 40.0 38.0 35.0 35-5

.~ ~ __.

25-5 26.8 23.4 24.7 21.5 23.5 22.8 24.7

Proc Instn Mech Engrs I96546 Vol I80 Pt 1

SPECIAL STEELS FOR HIGH TEMPERATURE SERVICE

60

50

VI v,

c IL

z" 30,

20.

1155

temperature impact properties of the two complex martensitic steels. This has not deterred designers from exploiting these steels with considerable success.

For turbine discs, hub temperatures have generally been in the region of 300-350°C where creep is not a problem and hub design has therefore been based on 0-1-0.15 per cent proof strength. Rim temperatures are higher, reaching up to about 550°C and in these cases creep properties must be considered in the design. One test criterion used extensively (3) is 0.15 per cent total plastic deformation in 100 h which has been derived by equating take-off and cruising conditions. Using this criterion, high temperature plots of the steels mentioned in this section are shown in Fig. 4.

In some of the later engines, hub temperatures have increased such that creep must also be considered at the hub position.

It will be noted from Fig. 4 that the later 12% Cr complex steel F.V.535 has more than 6 times the creep strength of the plain 12% Cr steel at 500°C. The same figure also shows that for the hub conditions (350°C) this same steel has a proof strength more than twice that of the warm worked austenitic steel.

Both the complex 12y4 Cr steels have been used extensively in aero-engines for turbine discs while the 12% CrMoVNb steel (F.V.448) also finds application as compressor discs and blading in aircraft gas turbines and for turbine blades and discs in industrial peak-load gas turbines. It is also used for bolts and valve parts in steam turbines. The longer time creep and rupture behaviour of this steel is discussed later in the lecture.

The major tonnage of the two special 12% Cr steels (F.V.448 and F.V.535) is now vacuum consumable arc remelted for aircraft gas turbine applications where maximum integrity is required. Vacuum remelting has little effect on creep properties but cleanness, transverse ductility and fatigue properties are greatly improved (3). Fig. 5 shows a histogram of hub transverse ductility results obtained on air and vacuum remelted compressor wheel disc forgings of F.V.535 steel. Since vacuum remelting was introduced, the hub transverse minimum

Nb Co

----_ 13/13 Cr NiG; Nb Co

G.18 B

i x i z c r L X

'!d'!%O 360 4bO 4;O 560 540 660 TEMPERATURE-%

Fig. 4. Disc materials, 0-15 per cent total plastic strain in 100 h

STEEL (2863 TESTS

COMPRESSOR WHEEL FORGINGS

800 -

500 - ul

W I-

LL

5400 -

0 0300- Z

200 -

O5 10 I 2u 15

- VACUUM- MELTED

STEEL 412 TEST9

1 20

- VACUUM. MELTED BILLET 195 TEST!

I 15

ELONGAT ION -per cent

Fig. 5. Comparison of air-melted versus vacuum-melted F. V.535-plain tensile elongations

elongation requirement has been increased from 10 to 15 per cent.

Vacuum remelting has also resulted in a marked improvement in the notch tensile strength, as can be seen from Fig. 6.

(152 TESTS)

% BELOW 100 - >50% -

..

100 120 60 80

,535 VACUUM-MELTED (540 TESTS)

lELOW 100 - < 0.2 %

TENSILE STRESS AT BASE OF NOTCH - todin2

Fig. 6. Notched tensile tests Proc Insrn Mech Engrs 1965-66 Vol180 Pt I

1156 H. W. KIRKBY

Reject No.

15

30

45

The benefits of vacuum remelting on rejection rates as determined by magnetic particle examination of finished machined discs can be seen from the data given in Table 5 which summarizes the results on air-melted and vacuum- remelted components examined to two standards-the original standard as applied to air-melted material and the new standard as applied to vacuum-remelted steel.

(2) 12% CrMoV steel From the development work carried out in connection with the disc materials for aircraft gas turbines, a third complex %-ton tensile stee1 was developed :

F.1.Mo.V. 0.12 12.0 0.75 0.60 0.20 per cent This composition is a further alloying variation of the plain 12% Cr and 12% CrMo steels mentioned earlier and it is currently being used in Great Britain for steam turbine blades, valve parts, etc. A similar steel with added tungsten has been used on the Continent for superheater tubes and cast turbine casings. Typical physical and mechanical properties obtained on the 12% CrMoV steel are shown in Table 6 which also gives data obtained on the plain 127; Cr and 1276 CrMo steels.

It is interesting to note that the development of these 12% Cr variants almost parallels that of the alloying of carbon steel for creep resistance, namely the CMo and MoV steels which have been used in the power industry for many years.

A comparison of the two series of steels using 100 000 h rupture data is made in Fig. 7. It will be noted that the plain 12% Cr steel is as strong as the CMo version and the 12% CrMo as strong as the MoV steel. Comparison on the basis of a limited deformation criterion, say 0.1 or 0-2 per cent, has shown that the two series of steels have similar strengths, i.e. plain 1276 Cr steel is equivalent to the plain carbon steel, CMo to the 12% CrMo and so on. This serves to emphasize the care needed in comparing the properties of steels as a change in criterion can affect the order of merit and indeed could well influence the selection of steels for service.

Both the plain 12% Cr steel and the molybdenum variant exhibit excellent creep ductility and are much superior to their non-stainless counterparts in this respect. Data so far accumulated suggest that these two stainless

C Cr Ni Mo V

Reject Reject Reject % No. Yo ___

0.70 584 27.3 5936

8.1 208 55.6 1380

1.79 792 31.5 7316

steels fracture in a transcrystalline manner* whereas carbon and carbon-molybdenum steels fracture with an intercrystalline habit. The contrast is most marked when comparing the 1276 CrMo with the CMo steel, the former showing a minimum creep ductility of about 15 per cent El (500-550°C) whereas the latter may fracture with a ductility as low as 1 per cent. The fracture habit of the 12% CrMoV steel is essentially intercrystalline and similar to that of the +"/o CrMoV pipe steel, The minimum creep ductility observed for the 12% CrMoV steel is around 6 per cent.

The three 12% Cr steels of this section are compared in Fig. 8 on the basis of 100 000 h to rupture for various temperatures, along with the 12% CrMoVNb turbine disc steel mentioned earlier. At 600°C the strength difference between the 12% CrMoV and the niobium-bearing types is small but the latter show increasing advantage a t lower temperatures.

Higher carbon variants (0.2 to 0.25 per cent) of the 12% CrMoV steel (usually with tungsten) are being used for rotor shafts on the Continent, and a 12% CrMoV steel with a small niobium addition is currently being manu- factured in the United States of America (4). In each instance superiority over the 'standard' 1% CrMoV steel is claimed and it is on this basis that studies have been made in Britain as to the relative merits of each steel for rotor shaft applications.

Rotors appear to be designed on the basis of 0.1 per cent creep deformation or some fraction of the rupture strength. If creep data are examined using a 0.1 to 0.2 per cent deformation criterion, there appears to be no advantage in using the 12% CrMoV steel in place of the 1% CrMoV steel. On the other hand, comparison on a rupture strength basis reveals the superiority of the 1276 CrMoV steel over the 1% CrMoV type. This can be seen from the data provided in Table 7 which gives the 100 000 h stresses for 0.1 and 0-2 per cent creep deformation and rupture strength for both the 12% CrMoV and 1% CrMoV rotor steels. Even with 50 per cent of the rupture strength, the 12% CrMoV offers a stress higher than that obtained when using the 0.1 or even 0.2 per cent deformation criterion. * A similar observation has been made in regard to the 9% CrMo

steel.

Reject No.

15

30

45

Table 5. Comparison of air-melted and vacuum-melted F. V.535 steel-magnetic particle examination

Reject Reject Reject % No. Yo ___

0.70 584 27.3 5936

8.1 208 55.6 1380

1.79 792 31.5 7316

Componcnt

0

19

19

Compressor wheels . .

Turbine discs . All parts . .

0 61 1-03

1.37 56 4.1

0.26 117 1.60

Vacuum-melted steel

AM standard I VM standard

No.

-I I I

Proc Insrn Mech Engrs 1965-66 Troll80 Pt 1


Table 6. 12% CrMoV steel-comparison with 12% Cr and 12% CrMo steels

Heat treatment I V I __

C Cr I Ni I Mo _ _ _ _ _ _ _ _ __ --

12y0 Cr (B.S. 3S61) (F.I.) . ' 0 10 125 1 0.25 I - - O.H. or A.C.lOOO"C, T.750"C 1276 CrMo (F.I.Mo.) . I 0-12 12.0 0 4 I 0.70 O.H. or A.C.lOOO"C, T.650"C

12.0 1 0.75 , 0.60 I 0% 1 O.H. or A.C.105O0C, T.700"C 12q4 CrMoV (F.1.Mo.V.) 0.12 * I

Physical properties: _________

I 127b Cr I 12% CrMo I 127i CrMoV

Specific gravity

Electrical resistivity at 20°C

20j4OO"C 20/500°C 20/6OO0C

7.73, equivalent to 482 lb/ft3 ~ 7.74, equivalent to 482.5 lb/ft3

56 microhm-cm 1 53 microhm-cm

0.000 011

0.000 01 1 1 0.000 010

0.000 0105 0.000 011

I 0.000 01 1 0.000 012 0.000 0115

0.000 012

Tensile properties :

I 0.05% 0.1% 0.2yA 0.5% ~ L.P. P.S. P.S. P.S. P.S. M.S. i

300°C 400°C 500°C 600°C

300°C 400°C 450°C 500°C 550°C 600°C

12% CrMoV (F.1.Mo.V.) R.T. 100°C 200°C 300°C 400°C 500°C 550°C 600°C 650°C

7.73, equivalent to 483 lb/ft3

59 microhm-cm

0.000 011

0.000 0115

0.000 012 0.000 012

9.5 11.4 10.0 7.5 7.0 7.1 1.03

22.0 20.2 20.0 15.2 15.0 14.6 14.0 8 .O 4.4

29.3 26.0 23.5 21.3 19.0 16.7 14.3 8.8 5.5

__

21.0 18.8 18.0 18.6 17.4 14.2 5.75

32.0 29.2 28.2 25.4 25.4 24.6 24.8 16.6 13.8

41.8 39.1 37.3 35.6 33.5 28.5 25.8 19.5 13.5

ton/in2 - __

23-2 20.6 19.8 20.4 18.8 16.2 7.4

33.9 31.6 30.6 28.0 28.0 27,4 27.2 20.4 16.2

43.2 40.5 38.7 37.3 35.3 30.8 28.0 22.0 15.8

____

25.1 22.1 21.4 21.4 20.1 17.6 9.25

35.8 336 32.2 30.4 30.0 29.5 29.2 23.2 18.3

44.2 41.6 39.8 38.5 36.8 32.5 29.7 24.2 17.9

26.9 23.4 23.5 22.7 21.3 19.3 11.3

38.2 36.0 33.8 33.0 32.2 31.6 30.8 25.6 20.4

45.0 42.5 41 .O 39.5 38.0 34.0 31-0 26.0 19.8

- ~~

39.3 1 34.5 1 32.3 32.1 1 29.9 24.9 19.8 1

I

46.2 I 43.3 40.0 38.4 ~

37.3 36.2 32.6 28.4

23'3 i 51.7 48.0 1 45.6 43.8 I 41.8 36.4 I 32-7 27.5 20.4

R.A. 01 /U

33 34 32 29 28 33 50

29 26 25 21 24 24 25 30 37

29 25 23 20 20 22 27 30 33

I

I

i

! i I 1

!

i i I

i

Izod, 1 Young's ft lb

72 79 76 74 74 77 90

73 73 68 67 73 74 77 84 86

68 70 73 65 68 71 78 84 86

I

-I-- 98

' 114 I 91 1 87

I 73 65

' 79

62 i 58 58

I

14 200 13 900 13 450 12 900 12 400 11 800 11 OOO

13 700 13 400 13 000 12 500 12 000

11 300

10 600

- -

14 500 14 200 13 680 13 200 12 700 12 050

11 250 -

-

STAINLESS AUSTENITIC STEELS The attractions of austenitic steels lie in their superior creep properties and resistance to oxidation compared with other classes of steel. On the other hand, austenitic steels are at a disadvantage in having higher thermal expansion, lower thermal conductivity and in many cases lower proof strength. The generally higher cost of these steels often constitutes a drawback. Proc Instn Afech Engrs 1965-66

Although austenitic steels have been used at elevated temperatures for many years, there was little in the way of intensive development of compositions for high temperature service until the introduction of the aircraft gas turbine and the later, post-war, large land and marine gas turbine projects. Both at home and abroad, many complex austenitic steels were then developed though few came to fruition in terms of wide practical application.

Vol I80 Pt 1

1158 H. W. KIRKBY

The role of the austenitic steel in aircraft gas turbines has already been touched upon and though this class of steel has been largely superseded by the stainless martensitic steel, the contribution made by the special austenitic steels was indeed great. Less certain is their value for large non-aircraft gas turbines as many difficulties were experienced with this type of power plant and few, if any, are now in operation.

20

LOW ALLOY 12% CHROMIUM LOW ALLOY 12% CHROMIUM

500'

1

Fig. 7. Comparison of low alloy and 12 per cent chromium steels 100 000 h rupture strength

24r

I 1 550 600

TEMPERATURE -'C

Fig. 8. 12 per cent chromium steels 100 000 h rupture strength

A further surge of interest followed the post-war steam turbine development when temperatures were raised to 1050°F and it was decided to pioneer the use of austenitic steels for superheater tubing, piping, headers, valves, etc. On the basis of the high temperature data available at the time, the 18/12/1 Nb steei (Type 347) was selected, a similar choice having been made earlier in the United States. In both countries considerable trouble was later experienced with weld junction cracking (5 ) in heavy weldments such as steam pipes and headers. Subsequent investigations showed that the cracking was caused by severe loss of creep ductility arising from a precipitation phenomenon* (6) (7) present to varying degrees in all niobium-bearing austenitic steels (and probably in some low alloy steels).

The reduction in ductility was found to be greatly intensified if the steel was subjected to high solution treatments as in welding, and the 18/12/1 Nb (Type 347) steel was particularly affected under these conditions. It was later concluded that cracking could be initiated by subsequent deformation in the range 550-850°C depend- ing upon the strain rate. This deformation could occur when cooling from welding under restraint, during stress relieving or in high temperature service, and the effects were cumulative. Whilst a subsequent heat treatment of 850-1050°C was found to reduce the effects of the strain- induced precipitation, it was difficult to be certain that cracking would not occur during cooling from welding or during heating up to 1050°C in stress relieving. In addition, although the longer time creep ductility was improved by the suggested post-welding treatment, it was found to be still quite low after several thousand hours' creep testing at 6O0-65O0C, values of the order of 1-2 per cent El being obtained.

Where heavy weldments are in question, the 18/12/l Nb (Type 347) steel is not now recommended though it has given satisfactory service for many years for such applications as superheater tubing where the section is relatively light.

As a result of the trouble with the l8/12/1 Nb steel, the case for continuing the use of austenitic steels in steam power plant received a serious setback but it did serve * Strain-induced precipitation of niobium carbide.

Table 7. 12% CrMoV and 1% CrMoV rotor steels-comparison of rupture strength and 0.1% and 0.2% total plastic strain in 100 000 h

I Steel I Temperature, Stresses for

-~ "C Rupture in I 5076 6076 ' 0.1q; ~ 0.2% 100 000 h rupture rupture I creep 1 creep I ~

lo/, CrMoV 12% CrMoV

I I 7

7 500 12-13 6-64 'i .8 6 i 500 i 15-16 i 75-8 i 9.6 i 6 ~

1 5 , 4:

6-6.6 4

7.2-7.8 l 4 1% CrMoV 525 10-11 12-13 I 525 I 12% CrMoV

lo/, CrMoV 550 8:-9 4 H f 5-54 3 3 t 1 550 1 9-93 1 44-42 1 5-:-5+ l 2 12% CrMoV

Proc Insrn Mech Engrs 1965-66 Vol180 Pr I

SPECIAL STEELS FOR HIGH TEiMPERATLJRE SERVICE 1159

to activate further the investigation of austenitic steels with better properties. More recent research work has followed two main lines: (1) better ductility, and (2) higher creep strength; and two types of matcrial were developed at The Brown-Firth Research Laboratories, namely :

F.V.555 (Type 316) 0-05 16.5 11 5 2.4 - 0.005 per cent F.V.548 0.08 16.5 11.5 1.5 1.0 0.005 per cent

The first steel F.V.555 (8) (9) is a selected range of the Type 316 steel with balanced chromium, nickel and molybdenum contents; an addition of boron is also made as it was found to contribute significantly to rupture strength. F.V.548 is a derivative of the 18/12/1 Nb steel with added molybdenum and boron. Both steels have 100 000 h rupture strengths superior to that ofthe lS/lZ/l Nb steel, as shown in Fig. 9.

This superiority also applies to the non-stabilized (Type 304) and titanium-stabilized (Type 321) austenitic steels which are also plotted in Fig. 9, these latter having similar rupture strengths to that of 18/12/1 Nb steel. This latter observation is interesting bccause 15 years ago the non-stabilized and titanium-stabilized steels were considered to be rather inferior in creep strength to the 18/12/1 Nb type. Since then considerably more testing has been carried out, particularly for longer times, and it is now accepted that there is little to choose between the three steels. Had this knowledge been available at the right time, it is possible that the cracking troubles with 18/12/1 Nb steel might never havc been experienced as it was only the apparent inferiority of the titanium-bearing steel which prevented it from being considered for use in 1050°F steam.

Besides being immune from strain-induced precipitation effects, the creep rupture properties of the F.V.555 (Type 316) steel are little influenced by heat treatment, particularly high solution treatments as experienced during welding. Creep ductility is excellent and therefore risks of cracking under creep conditions are small and no trouble in this respect has been experienced to date in F.V.555

C Cr Ni Mo Nb B

24

16/11 C r NI Mo ND -F.V 545

8-

0 1

TEMP~KATIJRE--"c 500 550 f.00 650 700 450

Fig. 9. 100 000 h rupture strength Proc I m t n Mech E1;611~ 196.5-66

5

headers, etc. which replaced some of the 18/12/1 Nb type which cracked in service. The creep ductility pattern can be seen in Fig. 10 which shows creep rupture-log time plots of tests carried out on actual steampipe materials at 650'C with comparison plots being given for the 1811211 Nb steel. The pattern of creep ductility behaviour of these steels at 600 and 700°C is similar to that shown at 650°C (Fig. 10).

The F.V.555 (Type 316) type of steel is to be used for piping, superheater tubing and steam chests, etc. in 1100°F supercritical power plant, and it is being used extensively for 1050°F power plant superheater tubing.

Being niobium-bearing, the F.V.548 steel has some susceptibility to the strain induced type of precipitation mentioned earlier, but the effect is less than that for the lS/lZ/l Nb steel (10). In addition, the general creep ductility behaviour of F.V.548 was found to be much superior to that experienced with the lS/lZ/l Nb steel hence the precipitation effect mentioned above has proved to be less important. Fig. 10 shows rupture strength and creep ductility plots for F.V.548 steel in comparison with the F.V.555 (Type 316) and the lS/lZ/l Nb steel. All the tests were carried out using testpieces cut from actual steampipes and the curves have been derived from test times of 30 000-50 000 h duration. Fig. 11 illustrates the savings in wall thickness which can be achieved by using the newer steels compared with l8/12/1 Nb steel.

F.V.548 has been used satisfactorily for several years for main steampiping and headers in one 1050°F supercritical steelworks power station. Table 8 lists typical physical and mechanical property data for both F.V.555 and F.V.548 steels.

A niobium-bearing steel of the same family and with similar properties, Esshete 1250 (11), is to be used for main steampiping etc. in a new 1100°F supercritical set in Britain.

As already mentioned, the trouble experienced with the

20 I

31 I

80

2 7°C + + ++ t : +

TIME- hour5

Fig. 10. Austenitic pipe steels-8 in bore 1; in wail, rupture at 650°C

Vo1180 Pt I

1160

55 48 48 49 47

H. W. KIRKBY

65 62 60 62 62

gave the following minimum percentage elongation values :

Comparison of wall thickness of pipes of various steels based on a common 8 in diameter bore and a pressure of 4000 lb/ina at 600"C, using mean diameter pipe formula S = P(D -t)/2t.

Fig. 11. Austenitic steels for steampipe service

lS/lZ/l Nb steel focused attention on the importance of creep ductility and since that time all new austenitic steels have been checked to determine whether creep ductility is seriously affected by high solution treatments as in welding, implemented with I tests on welded testpieces. Rupture tests carried out at 650°C for times up to about 10 000 h using high solution treatments of 1325-1350°C

Steel Solution treated Minimum elongation,

per cent

18/12/l Nb 1350°C 1 approximate 1350"C+stress-relieved 850°C 1 approximate

F . V .5 5 5 1350°C 15 (Tme 316) 1350"C+stress-relieved 850°C 20

F.V.348 ' 1325°C 7 1325°C +stress-relieved 850°C 13

It is clear from these results that the newer steels are less likely to give trouble with weld junction cracking because of their superior ductility and tests on welded testpieces support this finding.

HIGHER PROOF STRENGTH AUSTENITIC STEELS

As engineering materials, austenitic steels are often at a disadvantage on account of their low proof strength in applications where the latter becomes the deciding criterion such as at room temperature or at elevated temperatures below the creep range (say < 500°C). Various ways have been devised for increasing the hardness and proof strengths of this class of steel but not all methods are

Table 8. Stainless austenitic steels F. V.555 and F. V. 548-analysis, heat treatment and tensile properties

F.V.548 i F.V.555

I C Si Mn G Ni Mo Nb I C Si Mn Cr Ni Mo Nb

Heat treatment Physical properties

Specific gravity Electrical resistivity

at 20°C Coefficient of

thermal expansion 20/lOOT 20/300°C 20/5OO0C 20/700"C

0.08 0.40 1.0 16.5 11.5 1.5 1.0 A.C. 1150"C+3 h 850°C A.C.


73 microhm-cm

0.000 016 0.000 01 75 0.000 018 0.000 019

0.05 0.40 1.5 16-5 11.5 2.4 - A.C. 1050-1100°C


71 microhm-cm

0.000 016 0.000 0175 0.000 018 0.000 0185

Tensile properties (steampipe section):

I ton/in2

12.4 13.6 14.7 16.3 36.4 I 10.6 11.6 12.5 14.0 31.0 100°C R'T- 150°C 200'C I 300°C 400°C

~ 9.8 10.8 11.7 12.8 28.4

SOO~C 7.8 8.4 9.5 28.4 550°C 1 ;:A 7.6 8.2 9.2 28.0 600°C 7.5 8-2 8.8 9.8 26.6 650°C 6.8 7-4 7.9 8.9 25.6 700°C I 7.1 7.6 8.2 9.1 23.6

Proc Instn Mech Engrs 1965-66

E 0.050,& O.l0:6 0.2% 0.50,; P.S. P.S. P.S. P.S. M.S.

ton/in2 -___ _- - I --

55

49

39

39 39 39 40 41

55

52

49

46 45 47 51 50

13 150

- 11 510

10 420 10 200 9 970 9 720 9 390

14.1 14.9 15.5 16.6 34.8 10.1 10.9 113 123 29.2

9.2 9.9 10.3 11.2 28.5 8.6 9.3 9.8 10.4 27.9 8.2 8.8 9.1 9.9 28.0 8.0 8.5 8.9 9.4 27.0 7.3 7.7 8.2 8.9 24.3 7.3 7.6 8.0 8.6 22.0 ~.

7.5 7.8 8.2 6.8 2i.3 7.1 7.6 8.1 8.75 18.5

5 1 R.A. 0 1

/a /a

-.

Young's modulus, ton/in2

12 520 12 060

11 510 11 050 10 410 9 870

9 460 9 180 8 900

-

Vol180 Pt I


satisfactory, particularly where large pieces are required and where welding must be carried out. Two methods of current interest are warm working and nitrogen hardening, and these are discussed below.

Warm working Warm working is a recognized way of improving the strength of austenitic steels and has been used on and off for at least 20 years with varying success. For reproducible properties, the conditions of warm working, amount of deformation and subsequent heat treatment require close control and the problem is intensified when dealing with large areas and lengths of material, particularly in thick section. Closely controlled conditions are now obtainable at the new Plate Mill recently installed by Firth- Vickers Stainless Steels Ltd, and ‘double strength’ plates 2 metres wide x 10 metres long x 4-3 in thick are being produced by warm working with a guaranteed minimum 0.2 per cent proof strength of 25 ton/in2 at room temperature. This has the effect of reducing plate thickness by approximately a half. Welding tests show that although there is some local softening, the zone is very narrow and has little effect, as demonstrated by tensile and pressure vessel tests (12).

Fig. 12 gives various plots of 0.2 per cent proof stress (P.s.) and ultimate tensile strength (u.t.s.) versus temperature for 3 in thick titanium-stabilized l8j8 steel for welded and unwelded conditions with guaranteed minimum proof strengths at each temperature.

To take full advantage of the higher strengths available it is necessary to base the design on proof stress rather than on u.t.s. and in this respect Fig. 13 shows the proposed allowable design stresses derived on the basis of

o PARENT PI ATf n WLLDED PLATF

GUARANTEED ,,-<-.i- VALUES CF 3 2% PROOF STRESS

- - -____ --_ 18 MINIMUM

WELD&- Pi b T F t ,

‘4b ,b 100 I& 2bo A0 360 3:o T E M P E R A T U R E - - O C

Fig. 12. Effect of testing at elevated temperatures on the mechanical properties of high strength Staybrite FDP steel plate -1 in thick

Proc Instn Mech Engro 1965-66

Curve A Curve B

Curve C Curve D

TEMPERATURE-’C

Parent plate Jhigh strength plate (625 per cent of

Fig. 13. Proposed design stress for high strength Staybrite FDP plate

62.5 per cent of the guaranteed minimum 0.2 per cent proof stress* at various temperatures. The corresponding stresses using the A.S.M.E. Boiler Code for normal softened steel are also included in Fig. 13.

This method of improving proof strength is being applied to other grades of lSj8 steels, similar properties being obtained. No detrimental effects on other properties have been experienced so far, e.g. corrosion and stress corrosion resistance are unimpaired.

Warm-worked material has a high resistance to softening, temperatures up to at least 550°C for long times having little effect on the proof strength or other properties.

Nitrogen additions As shown in Fig. 14, additions of nitrogen to the 1818 austenitic type of steel result in a progressive increase in hardness and strength, this being accomplished without the need for special heat treatments. Up to 0.2 per cent nitrogen, no serious side effects are produced and at this level room temperature 0.2 per cent proof strengths of 20-25 ton/in2 are obtainable as against 13-14 ton/in2 for the normal low nitrogen product. In contrast to warm working, the strength of the nitrogen-bearing steels is inherent, i.e. it is not dependent on manipulation and therefore the attainment of the properties is not limited to certain forms, sizes and shapes as is the case with warm working. On the other hand, the proof strengths obtainable with the 0.2 per cent nitrogen limit are lower compared with those attainable with warm working, and the fall-off * The criterion in the draft I.S.O. Boiler Code.

Vol I80 Pt I

1162

701 €1. W. KIRKBY

t TENS!L

N 40

20 -

~E S T R E N G T H

PROOF STRESS PROSF STRESS

70t I

01 I I I I I I I 0 0 2 0.4 0.6

NITROGEN - p a r cant

Fig. 14. Effect of nitrogen on tensile strength of 181s austenitic steels

in proof strength with temperature is greater for the former. Also, compared with the warm worked steels 0.2 per cent p.s./u.t.s. ratios are lower for the nitrogen-bearing steels.

The gain in hardness and strength with added nitrogen is obtained on a wide variety of austenitic steel compositions including niobium-stabilized, non-stabilized and molybdenum-bearing grades, similar results being obtained for a given nitrogen level.

The influence of nitrogen has also been studied on the proof strength and creep behaviour of the two creep- resisting steels F.V.555 (Type 316) and F.V.548. Table 9 lists some short time tensile data obtained at three temperatures for both steels with high and low nitrogen

b F.V.548 with 0.12 per cent nitrogen.

Fig. 15. Effect of exposure at 650°C

contents, whilst Figs 15a and b show the effect of time at 650°C on the room temperature properties for the F.V.548 steel. These results show a useful gain in proof strength which is reasonably well maintained after 3 years' exposure at 650°C. Check hot tensile tests also confirm that the proof strength at temperature is not seriously affected by exposure at 650°C for long periods.

For service in the creep range, say upwards of 5OO0C, a high proof strength may not be important where, say, a 60 per cent rupture strength criterion is used in design except that the actual total deformation is likely to be less than that obtained with the low nitrogen counterpart.

Table 9. Stainless austenitic steels F. V.555 and F. V.548 with high and low nitrogen contents-short time tensile tests

I F.V.555 (Type 316)

C Ni Cr Mo N 1 0.05 10.70 16.70 2.30 0.033

0-20/, 0.5% 1.0% E R.A. Izod, ~ P.S. P.S. P.S. M.S. "/o f t lb

I ton/ina

13.6 14.4 15.4 36.4 67 78 110 8.0 8.8 10.0 29.0 50 73 -

600°C 6.3 6.9 8.0 25.7 45 69 -

C Ni Cr Mo N ~ 0.06 1257 17.34 2.63 0.129

~ 0.20/, 0-57; 1.0% E R.A. Izod, P.S. P.S. P.S. M.S. % % f t lb

20.1 20-6 22.1 44.4 54 65 90 12.2 13.0 14.2 36.7 45 64 -

600°C 10-3 11.2 12.2 32.7 42 55 -

Proc Instn Mech Engrs 1965-56

____

20°C 300°C 600°C

F.V.548

C Ni Cr M o Nb N 0.11 12.02 15.64 1.41 1.12 0.044

0.2% 0.5% 1.0% E R.A. Izod, P.S. P.S. P.S. M.S. yo "/o f t lb ______ I_

ton/in2

14.7 16.3 17.7 36.4 55 55 105 11.7 12.8 i4.6 28.4 39 49 - 8.8 9.8 11.3 26.6 39 47 -

C Ni Cr M o Nb N 0.09 11.55 15.84 1.38 0.92 0.153

0.274 0.5% 1.00,' /o E R.A. Izod, P.S. P.S. P.S. M.S. % f t lb

ton/in"

19-5 21.6 23.8 44-4 49 63 90 13.0 14.5 16.3 35.3 40 60 - 10.9 12.2 13.7 30.9 37 58 -

Vol I80 Pr I


Below the creep range, however, and as with warm-worked material, the proof strength can directly affect the allowable stress assuming the use of, say, 0-2 per cent proof strength as a criterion.

Much thought is being given to the most suitable design criteria for austenitic steels, one suggestion for certain applications being a 1 per cent total plastic deformation or a fraction thereof in place of a rupture strength criterion as used for pipes, tubes, headers, etc. This is another instance where a choice of criterion could not only influence the selection of steel but also alter the direction of metallurgical development.

The effect of nitrogen on the creep rupture strength is significant with the F.V.555 steel, as shown in Fig. 16. This figure shows a rupture strength scatter band obtained at 650°C for a number of F.V.555 type casts of steel with the high nitrogen test results superimposed. High nitrogen appears to ensure that the rupture strength of this type of steel lies at the top of the normal scatter band as obtained on boron-containing F.V.555 casts. More casts of nitrogen-bearing F.V.555 steel need to be tested before the full implications of the contribution of nitrogen can be defined, but the development is a promising one.

In contrast to the F.V.555 type of steel, nitrogen additions to F.V.548 do not appear to influence very markedly the rupture strength at 650°C.

TITANIUM-HARDENED AUSTENITIC STEELS There are several other methods for improving the hardness, strength and creep properties of austenitic steels, but most are unsatisfactory for one reason or another. One of the better known systems is that using titanium (and aluminium) as a hardener and this method has given rise to a wide range of precipitation-hardened materials from steels such as A.286* to the nimonic type of nickel-base alloys.

Fig. 17 illustrates the effect of titanium content on the hardness of 15-20 per cent chromium materials with * C Cr Ni Mo V Ti A1

A.286 0.04 15.0 26.0 1.3 0.3 2.2 0.2 per cent

P I

t

1 10 1co 1000 10000 lOd000

GURAT ION- hours

Fig. 16. Effect of nitrogen on creep rupture strength of F.V.555 at 650°C

Proc Insrrr -2fcch Er1Er.t 1965-66

4ooi

7-

0 05 1 0 1 5 2 0 2 5 3 0 3 5 TITANIUM-per cent

Heat treatment: l h 1050°C oil quench +16h 700'C air cool Fig. 17. Effect of titanium and nickel levels on the

hardness of an Fe, 15-200/, Cr, @lo//, C base

varying nickel contents, and shows that titanium levels in excess of 1-15 per cent are necessary in order to provide significant hardening.

It is not the intention to discuss this class of material in any great detail. Fig. 18 is of interest since it compares the rupture strength for 10 000 h of A.286 and nickel-base alloys with other steels. The superiority in rupture strength of these titanium-hardened materials is apparent, particularly with the nickel-base materials. Nickel-base materials have been used extensively in aircraft gas turbines for many years particularly for blading but the wheel is tending to turn full circle in that certain turbine discs are now being made in nickel-base or near nickel- base alloys which are really austenitic alloys. These latter materials combine high proof strength and high creep resistance with thermal expansion characteristics nearer to the martensitic steels than is the case with the original austenitic steel (G. 18B) mentioned earlier in the lecture.

Consideration has been given to the titanium-hardened alloys for long-life power plant applications and some of the nimonic alloys are seeing service for blading and bolt- ing in steam power plants and blading in non-aircraft gas

28r

G.18 6 0 500 550 600 650 700 750 ..

TEMPERATURE -'C

Fig. 18. 10 000 h rupture strength

Vol I80 Pt I

1164 H. W. KIRKBY

Exposure temperature

turbines. For applications involving welding of heavy sections, it is doubtful whether most of the (standard’ compositions would be satisfactory as high titanium contents give rise to low ductilities under high solution temperature welding conditions with a tendency to crack at weld junctions. Work is continuing in this field, however.

Duration 0.05% 0.1% 0.2% 0.5% U.T.S. 1 E R.A. I Izod, i L.P. P.S. P.S. P.S. P.S. 1 % ~ % 1 f t lb

I ton/in2 1

CAST AUSTENITIC STEEL PIPES As the result of the introduction of the I.C.I. reformer process and its subsequent adoption by the British gas authorities for making gas (13), many thousands of cast austenitic pipes and headers have been put into high temperature service. Two alloys which are higher carbon variations of well known wrought and cast austenitic steels are being used :

Type C Cr Ni 25/20 0.4 25 20 per cent 37/18 0.4 18 37 per cent

The main variation in composition from the ‘standard’ is the much higher carbon addition intended to increase high temperature strength and assist castability. The other attraction of cast austenitic pipes is the rather lower cost compared with the wrought counterparts. In addition, the high carbon cast structure adds t o the high temperature strength relative to the wrought counterpart.

The 25/20 alloy is being used mainly for the reformer piping which operates in the region of 800-950°C and this

Exposure tempcrature 0.05% 0.1% 0.2% 0.5% L.P. P.S. P.S. P.S. P.S.

i ton/in2

material has also been used for headers. The 37/18 alloy is generally being used for headers.

Fig. 19 compares the two alloys on a rupture strength basis with the standard low carbon wrought alloys, though wrought alloys with higher carbon are little stronger.

These high carbon alloys have fairly good high temperature strength but show variable creep ductility, some values being low particularly at weld junctions. The compositions are such that they tend to embrittle showing considerable loss of ductility at room temperature as shown in Table 10. Some failures, particularly in headers,

__ - ~ _ _ _

I

I

As cast

900°C

700°C

1 I f 0 ICOO 10030 100

DURATION -hours

Fig. 19

8.4 14.0 15.4 16.7 18.9 34.7 17 13 7, 7, 6 - - 6

3 months 10.3 15.3 16.9 18.6 22.0 4 4 6 months 9.6 14.7 16.3 18.2 21.5

3 months 9 -3 16.3 18.6 21.3 25.8 6 months 10.0 16.4 18.8 21.8

- 3 weeks 8.7 15.5 17.3 19.6 23.4 34.4 ~ 53

- 35.6

I

Table 10. Results of tensile tests on 25/20 and 37/18 cast austenitic steel pipes 6 in outer diameter x 2 in wall thickness 25/20 CrNi steel:

I I I

As cast

900°C

700°C

I 15.0 17.1 19-4 22-7 1 355 1 15 j 14 I 4, 4, 4 I

6 months 1 6.8 13.9 16.1 18.2 21-2 35.6

14.6 16.5 18.8 22.2 35.9 7 14.0 15.9 18.3 21-8 36.3 8

8.6 14.1 16.2 18.6 22.1 1 355 ~ 6 6 I


can be attributed to this form of embrittlement which is tending to encourage the use of lower carbon wrought pipes in places where temperatures are not so high and where failure can be catastrophic in terms of downtime as in the case of a header.

The large scale usage of these materials is an accomplished fact but data are not yet available on their long time behaviour in service. Until these data are available, the economic advantages (14) claimed for cast pipes are still a matter of conjecture.

In the meantime, the properties of the steels in question and of experimental steels are being studied and it is likely that new and better steels will become available as experience accumulates.

FINAL REMARKS In the foregoing pages, the author has endeavoured to highlight some of the developments made in the field of high temperature ferrous metallurgy; to bring out certain aspects, such as the creep ductility behaviour, of the various steels, and to illustrate the way in which different design criteria thinking can affect choice of steel. Attention has also been drawn to the efforts being made in regard to the collection of ‘scatter’ data and the long time creep and rupture testing of steels in order to provide the engineer with more reliable strength estimates. The author has also introduced a number of comparatively new steels, some of which were specially developed to meet new engineering requirements thus enabling new design con- cepts to be translated into reality.

In these various ways, some real and solid progress has been made since the 1939-45 war-probably more than at any other time. Even so, much remains to be done if the engineer is to make the most efficient use of special steels. For example, most of the creep and rupture data available and currently being accumulated are based on uniaxial laboratory tensile testing with only a small amount of work being carried out on notched specimens, p!essure testing of tubes and pipes, etc. Comparatively little is known of the effects of temperature cycling and superimposed fatigue stresses on the life of plants which have to run for 20-30 years. Even less is known about creep ductility phenomena. All these items are becoming urgent subjects for research- in both the technological and fundamental fields. I t is true that some attempt is now being made to measure creep deformation on actual components in practice, and this is certainly a step in the right direction.

From the metallurgical point of view, it is the author’s opinion that for some time to come the development of special steels will continue along the lines already touched upon in the Introduction to this lecture, namely, by a techndogical evolutionary process stimulated in the main

by engineering demand as hitherto. Steels will continue to emerge via the hard path of manufacturing and service experience with few, if any, arriving by any other route. The author is sufficiently optimistic, however, to believe that one day, albeit far off, the more fundamental knowledge will become available which will enable the metallurgist or his equivalent to predict theoretically a composition and heat treatment to give certain desired and novel properties.

ACKNOWLEDGEMENTS The research work carried out on the stainless steels reported in this lecture was done on behalf of Firth- Vickers Stainless Steels Ltd.

The author wishes to thank Mr R. J. Truman of The Brown-Firth Research Laboratories for his assistance in the preparation of this lecture.

APPENDIX R E F E R E N C E S

(I) HOPKIN, L. M. T., MURRAY, D. and DUVAL, D. ‘Creep properties of some butt welds in steampipes’, J . Iron Steel Inst. 1965 203, 819.

(2) Private communication. (3) DAVIES, W. T. ‘The user’s point of view’, Engineering

Materials and Design Symposium on Alloy and Stainless Steels, Buxton, March, 1962.

(4) BOYLE, C. J. and NEWHOUSE, D. ‘A new stainless steel for turbine rotors’, Metal Progr. 1965 87 (March), 61.

(5) ASBURY, F. E., .MITCHELL, B. and TOFT, L. H. ‘Cracking in weld joints of austenitic steel in C.E.G.B. power stations’, Welding Technology 1960.

(6) TRUMAN, R. J. and KIRKBY, H. W. ‘Some ductility aspects of 18/12/1 Nb steel’,J. Iron Steel Insr. 1960 196, 180.

(7) IRVINE, K. J., MURRAY, J. D. and PICKERING, F. B. “The effect of heat treatment and microstructure on the high- temperature ductility of 1811211 Nb steels’, ibid. 1960 196, 166.

(8) KIRKBY, H. W. ‘The creep properties of two molybdenum- bearing austenitic steels’, Alloy Metals Rev. 1959 9 (No. 92).

(9) TRUMAN, R. J., ROE, W. L. and MCWILLIAM, J. A. ‘An austenitic stainless steel suitable for high temperature steam pipework’, Brit. Weld. J . 1962 9 (May), 317.

‘An investigation of the creep ductility of lS/ l2/ l Nb and related steels’, Rep. on Structural Processes in Creep Symposium, Iron Steel Inst. Spec. Rep. No. 70, 1961, 204.

(11) Esshete 1250, Data Sheet No. 47, Engng Mater. Des. 1960 3 (Oct.), 647.

(12) MORLEY, J. I. and MCWILLIAM, J. A. ‘Double strength stainless steel plate’, ibid. 1965 8 (Sept.), 626.

(13) ANDREW, S. P. S. ‘Naphtha reforming for synthesis gas and town gas production’, Paper to Ann. Gen. Meeting, SOC. Chem. Industr., Manchester, July 1964.

‘Alloy steels in a reforming furnace of a chemical plant’, Iron Steel Inst. Spec. Rep. No. 86.

(10) KIRKBY, H. W. and TRUMAN, R. J.

(14) EDELEANU, C. and ESTRUCH, B.

Proc Instn Mech Engrs 1965-66 Vol180 rt 1

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