METALS AND MATERIALS International, Vol. 7, No. 4 (2001), pp. 297~302

The Effect of W and N Addition on the Mechanical Propertiesof 10Cr Steels

Sung Ho Kim, B. J. Song and Woo Seog Ryu

Nuclear Materials Development Team, Korea Atomic Energy Research InstituteP.O. Box 105, Yusong-ku, Tajon 305-600, Korea

The effect of W and N on the creep properties and microstructural degradation in 10Cr steels was studied.Creep testing was performed to determine the creep rupture strength and minimum creep rate. Transmissionelectron microscopy was used to observe the microstructural degradation during the creep deformation. Wand N which were added to the 10Cr steel increased the creep rupture strength and decreased the minimumcreep rate. As W and N were added, the thermal stability of the subgrain and carbide was improved, thus thegrowth of the subgrain and carbide during creep deformation was restricted. In W added steel, the Lavesphase played an important role in increasing creep rupture strength. But the impact toughness was rapidlydegraded by the addition of W after aging at 600oC for 5000 hours. So one must evaluate more accuratelythe effect of the Laves phase on long term creep and impact properties. In N added steel, V(C, N) wasprecipitated in the lath boundary and within the lath. The size of the precipitates was 20-50 nm. The increaseof creep rupture strength in N added steel may be due to the precipitate of the V(C, N). Future tests arerequired to clarify the effect of N on creep and impact properties.

Keywords : Cr steel, creep rupture strength, toughness, tungsten, nitrogen

1. INTRODUCTION

High Cr steels have been widely used in power plants andthe chemical and petroleum industries. Especially high Crsteels have been receiving attention for fuel cladding and ductapplications in the core components of the liquid metalreactor because of their excellent irradiation swelling re-sistance [1,2]. These steels have also been used in liquidmetal reactor steam generator tubes since they have lowercarbon activity, good waterside corrosion resistance, resis-tance to waterside stress corrosion cracking in the event ofchloride caustic ingress and resistance to stress corrosioncracking following a steam to sodium leak. One of the mostimportant challenges in fusion technology research anddevelopment is the development of low activation materials.Presently ferritic-martensitic steels, vanadium alloys and SiC/SiC composite materials are considered as promising candidates.Of these candidates, low activation ferritic-martensitic steelsare recognized as the most advanced and mature materials [3,4]. But the creep rupture strength of ferritic-martensitic steelsfalls rapidly when exposed at temperatures of 600oC andgreater. In order to increase operating temperature and powerplant time, an optimum balance between fracture toughness

and creep rupture strength should be achieved.Tungsten is a ferrite stabilizing element and acts as a solid

solution hardening element. The addition of W retards therecovery of the dislocation and recrystallization rate whiletempering since W inhibits the diffusion of iron atoms [5]. Wmay also dissolve into the M23C6 carbides, thus increasing thethermal stability of the carbides and high temperature long-term creep rupture strength [6]. Nitrogen is a austenitestabilizing element and precipitated as nitride. The thermalstability of nitride is superior to the carbide because theenthalpy of formation of nitride is higher than that of carbide.The content of nitride that is more stable increases as thenitrogen content increases. So the movement of free dis-location may be effectively inhibited by the addition of N [7].

In the present work, steels that each contains more W ornitrogen have been studied to evaluate the effect of W andnitrogen on creep rupture strength and impact toughness in10Cr steels.

2. EXPERIMENTAL PROCEDURE

The chemical composition of 10Cr steels investigated inthis study is given in Table 1. The alloying change from

This article is based on a presentation made in the “Symposium on Nuclear Materials and Fuel 2000”, held at the Korea Atomic Energy ResearchInstitute (KAERI), Taejon, Korea, August 24-25 under the auspices of the Ministry of Science and Technology (MOST).

��� Sung Ho Kim et al.

10CrW to 10Cr is the reduction of the Mo content andaddition of W. The nitrogen content in 10CrN steel isincreased from 0.02 wt.% to 0.05 wt.%. These steels werelaboratory melted in a vacuum in an induction furnace. Three30 kg ingots were melted and hot rolled at 1150oC to a finalplate thickness of 15 mm. Heat treatment was carried out in avacuum furnace. The heat treatment consisted of austeni-tizing at 1050oC for one hour followed by air cooling andtempering at 750oC for two hours also followed by aircooling. The gage length and diameter of the creep testspecimen is 30 mm and 6 mm, respectively. The creep testwas performed at 600oC under constant load conditions. Theelongation of the specimen was measured with LVDT(Linear Variable Differential Transformers). The microstruc-ture was observed with TEM (Transmission Electron Mic-roscopy). The specimens for TEM investigation were preparedwith a 25% HNO3-metnanol solution and electropolished toperforation in a 10% HCl-methanol mixture at 40oC. Toughnessin the as-tempered condition and after aging at 600oC for5000 hours was examined with Charpy impact testing. TheCharpy V-notch impact tests were carried out between -120oCand 180oC according to the ASTM E23. Standard Charpy V-notch impact test specimens were machined with LTorientation (notch vertical to the rolling direction).

3. RESULTS AND DISCUSSION

3.1. Creep properties

Fig. 1 shows the creep rupture strength of high Cr steelscrept at 600oC. The W added steel showed the highest creeprupture strength. The creep rupture strength of W added steelwas 30 MPa higher than that of 10Cr steel in the same time torupture. Creep rupture strength was also considerably imp-roved by the increase of nitrogen content.

Fig. 2 shows the relation between minimum creep rateversus stress. The minimum creep rate of W added steel hadthe lowest value under the same creep stress and temperature.10Cr steel had the highest minimum creep rate. Eventuallyminimum creep rate was decreased by the addition of W andnitrogen, thus the creep rupture strength was increased.

The relation between minimum creep rate and stress is asfollows in the same creep temperature:

where A is a material constant and n is a stress exponent orcreep exponent. Creep mechanism is presumed by the nvalue. In the present work, the n value was above 20 whichcould be considered as a very high value. Weertman [8] cal-culated the n value as 4.5 under the assumption that creep ratedepends on the dislocation climb. But the n value wasobtained mainly as 3 from the calculation under a differentassumption. Experimental results showed that the n valuewas 3 only in solid solution alloy, being above 5 in pure metaland single phase alloy. The n value of Cr-Mo steel especially

ε′m Aσn=

Table 1. Chemical composition of 10Cr steels (wt.%)

C Si Mn Ni Cr Mo V Nb W N

10Cr 0.15 0.10 0.45 0.46 9.79 1.23 0.20 0.18 - 0.0210CrW 0.18 0.09 0.47 0.42 9.87 0.49 0.20 0.20 2.01 0.0210CrN 0.15 0.08 0.48 0.50 10.0 1.28 0.20 0.20 - 0.05

Fig. 1. Creep rupture strengths of 10Cr steels at 600oC.

Fig. 2. Minimum creep rate plotted against creep rupture strength for10Cr steels.

The Effect of W and N Addition on the Mechanical Properties of 10Cr Steels ���

was 19 [9]. So back stress is introduced to explain the dis-agreement. Derby and Ashby [10] showed that n value maybe above 5 considering the threshold stress of the material.The n value is changed by the thermal stability of the precip-itates and the maintenance of the solid solution hardeningeffect during creep.

Fig. 3 shows the creep rupture strength versus the Larson-Miller parameter. Three steels showed a similar tendency.The 105 hours creep rupture strength that is predicted fromthe Larson-Miller parameter was about 124 MPa in W addedsteel, 116 MPa in nitrogen added steel, and 85 MPa in 10Crsteel. It is anticipated that the creep rupture strength wasgreatly improved by the addition of W and nitrogen. Moreexperimental tests are necessary to predict more accuratelythe effect of W and nitrogen on creep rupture strength.

3.2. Change of microstructure after creepFig. 4 shows the microstructure that was observed with

TEM after it had crept at 600oC for approximately 1000hours. The microstructure before creep deformation did notshow any difference, that is, the prior austenite grain size,martensite lathe width, and precipitate size were nearly samein the three steels. Afterwards creep deformation sub-grains formed in the 10Cr steel, but the martensite lathstructure was preserved in the W and nitrogen added steelsafter creep deformation.

Fig. 5 shows the microstructures of precipitates after creeptesting for about 1000 hours. The major precipitates were theM23C6 carbides in the tempered state. The creep deformationaccelerated the growth of the precipitates. The growth of theprecipitates was not severe in 10CrW steel, showing that theaddition of W increased the thermal stability of the precipi-tates by the substitution of Fe atoms for W atoms in the M23C6

carbide. Another change in precipitates is the precipitation ofthe Laves phase. A Laves phase with a 100-200 nm size wasprecipitated and effectively restricted the movement ofdislocation during creep deformation. The precipitation of theLaves phase was accelerated by the addition of W, so a largeramount of Laves phase was precipitated in W added steel.

The Laves phase formed was about 100 nm in size in theearly stages of creep deformation. These were grown to 500-1000 nm in long term creep [11,12].

The M23C6 carbides initially grew faster than the Lavesphase, but as the Cr in the matrix is depleted, the growth rateof M23C6 carbides falls more rapidly than that of the Lavesphase. Eventually, the M23C6 carbides will stop growing, butthe Laves phase will continue to grow since there remainsexcess Mo and W in the matrix. The growth of the Lavesphase will continue until all the excess solute is removedfrom the matrix. This leads to the distribution of widelyspaced, large particles at long time intervals. The Lavesphases of about 100-200 nm in size had a beneficial effect oncreep rupture strength. But when the Laves phases coarsensto above 1000 nm, the creep rupture strength is bound to comedown [7].

Creep rupture in tempered martensite steels is usuallycontrolled by cavity nucleation and growth. Cavity growth isconstrained in these materials and cannot proceed without thesimultaneous creep of the surrounding matrix. The particlesin creeping solids influence the creep rate due to theirinteraction with free dislocation arrangements such as sub-grain boundaries. Thus, creep rupture life strongly dependson the microstructural features of the particle population.Subgrains constitute hard regions which represent obstaclesagainst dislocation motion. The precipitates stabilize thesubgrain boundary network. That is, the precipitates restrictthe motion of dislocation, and are grown by the dislocationpipe diffusion. So the recovery of dislocation and the growthof precipitates have a close relationship. In the present work,the improvement of creep rupture strength in W added steelwas due to: (i) delay of the softening of the matrix byrestraining the growth of precipitates and subgrains; and (ii)inhibition of the movement of dislocation by the precipitationof the Laves phases. Coarsened Laves phases had no moreprecipitation hardening effect and Laves phases precipitationremoving Mo and W from solid solution should be regardedas detrimental to creep rupture strength. So the strengtheningeffect by the Laves phases will disappear in long term creepdeformation. The effect of the Laves phase on long termcreep performance is unknown. Future studies on the effect ofthe Laves phase are required.

Carbonitrides will be precipitated instead of carbides in Nadded steel. The enthalpy of the formation of carbonitrides ishigher than that of carbide, so the thermal stability ofcarbonitrides will be superior to that of carbides during longterm service at high temperature. As shown in Fig. 6, many

Fig. 3. Creep rupture strength property plotted against Larson-Millerparameter for 10Cr steels.

��� Sung Ho Kim et al.

V(C, N) carbonitrides were precipitated along lath boundariesand within the lath. The size of V(C, N) carbonitrides wasabout 5-20 nm. These fine precipitates will effectively preventdislocation movement. Eventually, in nitrogen added steel, theimprovement of creep rupture strength was due to the formationof carbonitrides which were thermally stable and fine.

3.3. Impact propertiesFig. 7 shows the temperature dependence of the impact

energy of high Cr steels after tempering treatment. The uppershelf energy was 225 J in 10Cr steel and 168 J in W added

10CrW and nitrogen added 10CrN steels. The ductile-to-brittle transition temperature at an impact energy of 68 J wasabout -65oC in 10Cr steel and -30oC in 10CrW and 10CrNsteels. V. K. Sikka [13] proposed a guideline of impactproperties in high Cr steels: the upper shelf energy must beat least 136 J and the ductile-to-brittle transition temperature atan impact energy of 68 J should be less than 10oC. All steelssatisfied the guideline in the tempered state.

To investigate the change of the absorbed energy after longtime aging, the specimens were aged at 600oC for 5000 hours.The impact test results are shown in Table 2. The absorbed

Fig. 4. TEM micrographs of (a, b) 10Cr, (c, d) 10CrW, and (e, f) 10CrN. (a, c, e) as tempered and (b, d, f) after crept at 600oC for 1000 hrs.

The Effect of W and N Addition on the Mechanical Properties of 10Cr Steels ���

energy was 147 J before aging but abruptly decreased to 72 Jafter aging in W added steel. On the other hand, the absorbedenergies of 10Cr and 10CrN steels changed from 204 J to 176 Jand from 152 J to 133 J respectively, with aging. Thedecrease of the absorbed energy in W added steel after agingwas much larger than that in nitrogen added steel. As shownin Fig. 5, a large quantity of Laves phase was precipitatedduring creep deformation in W added steel. The decrease ofabsorbed energy in W added steel after aging may be due tothe precipitation of the Laves phase. Later on, the effect of the

Laves phase on long term creep and impact properties shouldbe evaluated more accurately.

4. CONCLUSIONS

The creep rupture strength and impact properties of 10Crsteels containing larger amounts of W and nitrogen wereinvestigated. The following conclusions were obtained:

Fig. 5. Precipitates morphology of (a) 10Cr, (b) 10CrW and (c) 10CrNafter creep at 600oC for 1000 hrs.

Fig. 6. TEM micrograph showing V(C, N) precipitates in 10CrN steel.

Fig. 7. Transition curves for charpy impact specimens of steels aftertempering.

Table 2. Charpy impact test results after aging at 600oC for 5000hrs

Before Aging After Aging

10Cr 204 J 176 J10CrW 147J 72 J10CrN 152J 133 J

��� Sung Ho Kim et al.

(1) The creep rupture strength of 10Cr steels was improvedwith the addition of W and nitrogen. The minimum creep ratewas also decreased with the addition of W and nitrogen.

(2) The thermal stability of the microstructure was increasedin W and nitrogen added steels. Thus, the growth of precipi-tates and martensite lath were restricted. The Laves phases,especially in sizes of 100-200 nm,were precipitated duringcreep deformation and effectively restricted the movement ofdislocation in W added steel.

(3) In W added steel, the Laves phases had an importantrole in increasing the creep rupture strength. But the impacttoughness was rapidly degraded by the addition of W afteraging at 600oC for 5000 hours. So a more accurate evaluationof the effect of the Laves phase on long term creep andimpact properties is required.

(4) In N added steel, V(C, N) was precipitated on the lathboundary and within the lath. The sizes of the precipitateswere 20-50 nm. The increase of creep rupture strength in Nadded steel may be due to the precipitate of the V(C, N).Future tests are required to clarify more exactly the effect ofN on creep and impact properties.

ACKNOWLEDGMENT

This work has been carried out as a part of the reactor corematerials research and integrated database establishmentunder the nuclear R&D program of the Korean Ministry ofScience and Technology.

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