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CIRP Annals - Manufacturing Technology 61 (2012) 543–546

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Cryogenic deep rolling – An energy based approach for enhanced coldsurface hardening

Daniel Meyer

Foundation Institute of Materials Science, Department of Manufacturing Technologies, Badgasteiner Str. 3, Bremen 28359, Germany

Submitted by Ekkard Brinksmeier (1), Bremen, Germany

1. Introduction

From industry it is known that any method to integrate surfacehardening in a production line is highly desirable. The advanced,heat treatment-free production line ‘‘cold surface hardening’’ [1] isnew and very challenging. It allows for process-integrated surfacehardening by achieving martensitic transformation of the surfaceand subsurface layers caused by purely mechanical loads. Unlikethermal, thermomechanical, and thermochemical methods, thehardening step can be performed in the machine tool immediatelyafter soft machining e.g. by a deep rolling process [2]. Whereas mostmechanical processes [3–5] are known to increase the surfaceintegrity by inducing compressive residual stresses and strainhardening, cold surface hardening is based on microstructuraltransformation. Precondition for the application of the advancedproduction line is the presence of high amounts of retainedmetastable austenite in the workpiece material. This paper aimson enabling of the application of very stable microstructures, whichwould come along with invaluable advantages in soft machining andthe functional performance of cold surface hardened components.

1.1. Mechanically induced martensitic transformation

According to Vohringer and Macherauch [6] as well as Tamura[7], microstructural transformation usually occurs as a specific

the material. Cold surface hardening is based on the idea to apmetastable austenitic microstructures, which show a cer(thermally induced) DGRTtherm below the activation energyroom temperature (RT). To achieve a total DG which equals

activation energy, the additional energy required for inducingmartensitic transformation DGRTmech is applied by purely mechical processes such as deep rolling. However, cold surhardening in its conventional state showed one major limitatwhich is the narrow spectrum of microstructures suitableapplication in the advanced production line. The matershowing sufficient martensitic transformation in the surface

subsurface layers were very instable, showed moderate machability and carried the risk of undesired transformation duringservice life of the components.

1.2. Energy based approach to overcome limitations

Generally, there are two possibilities to make more stamicrostructures available for application in cold surface haening: (i) further increase of the mechanical loads or (ii) reducof the workpiece temperature. As the application of very hmechanical loads can lead to material fatigue and the maximloads are limited by the machine tool, the most promisapproach is to combine thermal and mechanical effects to increthe total DG.

A R T I C L E I N F O

Keywords:

Surface

Hardening

Cryogenic

A B S T R A C T

In an advanced production line including cold surface hardening, martensitic transformation

metastable austenites as workpiece material was achieved by deep rolling. The deep rolling pro

replaces energy- and cost-intensive heat treatments. Cryogenic deep rolling is a novel approac

combine two different sources of energy (thermal and mechanical effects) and thereby allows

application of more stable microstructures. This comes along with advantages regarding soft machi

and the stability of the core material during a component’s functional loading. This paper presents

theoretical background, suitable parameters, and the resulting surface integrity of cryogenic deep ro

components and gives clear recommendations for industrial application.

� 2012 C

Contents lists available at SciVerse ScienceDirect

CIRP Annals - Manufacturing Technology

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activation energy is achieved due to thermal effects. Starting at atemperature, at which both phases are thermodynamicallybalanced (T0), reducing the temperature leads to an increasingabsolute value of the energy delta (DG) between austenite andmartensite (Fig. 1). Though martensite would be thermodynami-cally favorable, transformation from austenite to martensite doesnot occur until the temperature reaches a level (martensite-starttemperature MS) where DG equals the specific activation energy of

0007-8506/$ – see front matter � 2012 CIRP.

http://dx.doi.org/10.1016/j.cirp.2012.03.102

As shown in Fig. 1, the lowering of the workpiece temperatfrom RT to cryogenic conditions (T1) reduces the amount of

energy, which has to be induced mechanically. Consequenunder cryogenic conditions a higher total value of DG should refrom applying a constant amount of mechanical load compareRT. The combination of thermally and mechanically induenergy thus should allow for martensitic transformation in mstable microstructures.

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D. Meyer / CIRP Annals - Manufacturing Technology 61 (2012) 543–546544

Process-integrated cryogenic treatment

rocess-integrated cooling of the workpiece is a well estab-d method in cryogenic machining. Liquid nitrogen and CO2-

are used to reduce the temperature of the contact zone.ara and Kumagai described an increase of surface quality wheng liquid nitrogen as a coolant in machining of austenitic steels

hereas Evans utilized the low temperatures to successfullyce chemical wear in diamond turning of steel [9]. The reducedr of the diamond tool using CO2 as a coolant was furthermoreribed by Abele and Schramm [10].

n more recent studies, the surface integrity resulting fromgenic machining regarding residual stresses and hardness areessed by Pusavec et al. [11]. Pu et al. [12] analyzed theence of the combination of process-integrated cryogenic

tment and mechanical processes (burnishing) on the grain sizeg-alloys. They revealed a significant grain refinement and thus

eased corrosion resistance.ryogenic deep rolling of metastable austenitic steels to achievee comprehensive martensitic transformation in surface hard-g is a new approach to enhance the production line of coldace hardening.

xperimental setup

Heat treatment and premachining

he stability of the austenitic microstructure of cylindricalples with an initial diameter of 60.8 mm was adjusted to fivels by varying the austenitizing temperature TA of a heattment. In a range of TA = 1105 8C to TA = 1185 8C, theenitizing temperature was increased by 20 8C each. To alloweneration of microstructures with high amounts of retained

enite, highly alloyed high-carbon steels have to be applied. Incase X210Cr12 (AISI D3) was used as a workpiece material.he samples were premachined identically by longitudinaling at a CNC lathe using the parameters given in Table 1. CBNs were used as the microstructures resulting from austenitiza-

at lower temperatures showed high hardness.

Initial properties of the workpieces

whereas cooling down the most stable heat-treatment variant(TA = 1185 8C) to �170 8C did not cause martensitic transformation.

The stability of the microstructure against purely mechanicaleffects was analyzed by tensile tests. The effective stress inducingthe martensitic transformation sA rose from 300 MPa(TA = 1105 8C) to 740 MPa (TA = 1145 8C). For austenitizing tem-peratures above TA = 1145 8C no martensitic transformation wasobtained until the break of the test specimens.

2.3. Deep rolling experiments

Deep rolling was performed at the same CNC lathe as thepremachining using a hydraulically supported deep rolling tool. Toreduce the workpiece temperature in the experiments oncryogenic deep rolling, CO2-snow cooling was applied. Besidesthe parameters chosen for the experiments, Fig. 2 shows thearrangement of the CO2-snow-nozzle and the deep rolling tool.Temperature measurements at a rotating workpiece indicatedtemperatures below �30 8C after the chosen time for pre-cooling.

3. Resulting surface integrity

To verify that the energy based approach to combine themechanical and thermal effects of cryogenic deep rolling allows formore comprehensive martensitic transformation, the contents ofmartensite were detected by a calibrated inductive measurementdevice (feritscope). Its function principle is based on the changes ofpermeability due to microstructural changes. Fig. 3 summarizesthe contents of martensite in the surface and subsurface layersafter deep rolling and cryogenic deep rolling metastable austeniteswith varied microstructural stability.

All cryogenically deep rolled samples show higher amounts ofmartensite compared to those samples, which were deep rolledapplying the same mechanical loads at room temperature. Asexpected, the content of formed martensite decreases with

Gib

bs f

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temperature TMs T0RT

Martensi te α‘

Austenite γ

ΔGMs ΔGRTthe rm

ΔGRTmech

T1

ΔGT1therm

ΔGT1mech

ΔGtherm+ ΔGmech = ΔGMs= Activation en ergy

1. Schematic illustration of the dependency of the Gibbs free energy on

erature for austenitic and martensitic microstructures.

Table 1Cutting parameters for premachining by longitudinal turning.

Tool CNMA 120404 F (CBN) tool orthogonal

clearance a = 08, tool orthogonal rake

angle g = 08, corner radius re = 0.4 mm

Parameter Value

Cutting speed vc 80 m/min

Depth of cut ap 0.2 mm

Feed f 0.2 mm

Metalworking fluid Dry

Fig. 2. Experimental setup for cryogenic deep rolling.

fter heat-treatment the hardness of the material varied fromRC (TA = 1185 8C) to 55 HRC (TA = 1105 8C). The initial contenttained austenite RA was determined using X-ray diffraction) and was found to be in between 88% (TA = 1105 8C) and 97%

1185 8C). Due to the high amount of carbides in the material, apletely austenitic microstructure cannot be obtained.n dilatometer tests, the MS-temperatures of the differentostructures were assessed to describe the stability of the

ants against purely thermal effects. The lowest austenitizingperature (TA = 1105 8C) lead to a MS-temperature of 10 8C

increasing austenitizing temperature. This is due to the higherstability of those microstructures (see Section 2.2).

From Fig. 2 it can be derived that the heat-treatment variantsTA = 1165 8C and TA = 1185 8C are too stable to allow forcomprehensive martensitic transformation caused by deep rolling.As the microstructure showing an MS-temperature of 10 8C(TA = 1105 8C) is too instable to avoid undesired martensitictransformation during the lifetime of a component, the mostpromising variants for application in cold surface hardening areTA = 1125 8C and TA = 1145 8C. In Fig. 4 the roughness values ofthese variants resulting from deep rolling under varied conditions

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D. Meyer / CIRP Annals - Manufacturing Technology 61 (2012) 543–546 545

are given. Comparing the roughness after deep rolling at roomtemperature and under cryogenic conditions, a slight increase ofsurface roughness can be revealed, when CO2-snow cooling isapplied. This can be explained by minor embrittlement of thematerial at lower temperatures and thus by a decreased plasticity.

Independent from the thermal conditions, both microstructuresshowed a significant increase of hardness in the subsurface layersafter deep rolling. In Fig. 5, the hardness profiles after turning, deeprolling at room temperature and cryogenic deep rolling arecompared for the more stable microstructure.

Due to the thermomechanical loads during turning, a slightincrease of hardness can be obtained in depths of up to 0.2 mm.Deep rolling at room temperature results in higher hardness and anincreased hardness penetration depth. Cryogenic deep rollingleads to the highest hardness values and the most comprehensivedepth effect. Due to the characteristics of Hertzian stress, themaximum hardness values after deep rolling are found below thesurface (z = 0.2–0.3 mm).

Depth profiles of the content of retained austenite (Fig. 6) wereassessed by XRD using CrKa-radiation and stepwise electrolyticalremoval of thin layers. They reveal that the thermal conditions donot have an effect on the amount of martensitic transformation atthe surface. In depths below 0.1 mm, the combination of thermaland mechanical effects during cryogenic deep rolling leads to afurther decrease of content of retained austenite. In both cases, thecontent of retained austenite is reduced in depths below 1.4 mmcompared to the initial state.

4. Discussion and summary

The presented results give proof of the high potential of

energy based approach to combine the thermal and mechaneffects of cryogenic deep rolling to achieve more comprehenmartensitic transformation of metastable austenites. The appltion of more stable microstructures in the advanced production

of cold surface hardening has some significant advantages.

hardness depth profiles shown in Fig. 7 demonstrate thatdrawback regarding the hardening effect is visible when cryogcally deep rolling the more stable microstructure compared to drolling of the more instable microstructure at room temperatu

The interrelationships between the chosen parameters andresulting surface and subsurface properties are valid forinvestigated microstructures.

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ensite

MA

10

90

40

60

50

70

0

20

30

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austeni tizing tempe rature TA

110 5 1125 1145 oC 1185

db = 6 mm , Fr = 1130 N, RT

db = 6 mm , Fr = 1130 N, CO2-sno w

db = 13 mm , Fr = 53 09, RT

db = 13 mm , Fr = 53 09 N, CO2-snow

max/min of 5 measurements

Fig. 3. Contents of martensite in the surface and subsurface layers after deep rolling

under varied mechanical loads and thermal conditions.

µm

8

ess

Ra

, R

z

6

14

10

TA = 1125° C TA = 114 5°C

Ra

Rz

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0 0.5 1.0 1.5 2.0 2.5 mm 0

200

400

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800

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ness

HV

0.5

600

depth below the surface z

1200TA = 114 5°C, db = 13 mm, Fr = 530 9 N

deep rolled (RT)

turned

cryogeni call y dee p rolle d

Fig. 5. Hardness profiles after turning and deep rolling under varied the

conditions (TA = 1145 8C, db = 13 mm, Fr = 5309 N).

Fig. 6. Contents of retained austenite and micrograph after deep rolling u

varied thermal conditions (TA = 1145 8C, db = 13 mm, Fr = 5309 N).

ess

for be

fterling

therce,

0

2

4

rou

gh

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Fr = 1130 N Fr = 5309 N

RT Cryo RT Cryo

turn

ed

db = 6 mm db = 13 mm

Fr = 1130 N Fr = 5309 N

RT Cryo RT Cryo

turn

ed

Fig. 4. Surface roughness after deep rolling under varied mechanical loads and

thermal conditions.

� Big ball diameters and high rolling pressure cause high hardnpenetration depth and low roughness values.� In case the geometry of a component does not allow

application of big ball diameters, the rolling pressure has toincreased to keep the rolling force constant.� CO2-snow peening results in a slightly increased roughness a

deep rolling. Independentfromthethermalconditions,deep rolinduces high compressive residual stresses (not shown here).

Analyzing the hardness penetration depths resulting fromdeep rolling experiments as a function of the applied rolling fo

allowrecoleadroomtempenetemexpesepa

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D. Meyer / CIRP Annals - Manufacturing Technology 61 (2012) 543–546546

s for separation of mechanical and thermal effects. Besidesnfirming that at identical rolling forces cryogenic deep rollings to higher hardness penetration depths than deep rolling at

temperature, Fig. 8 indicates how variation of theperature and the rolling force influences the hardnesstration depth. As previous studies on deep rolling at room

perature [13] demonstrated, the results from deep rollingriments with varying ball diameters have to be discussedrately.egarding the hardness penetration depth, the results from

rolling the more stable variant using a 13 mm ball diameter, to remarkable results. Independent from the thermalitions, an increase of the rolling force from 2655 N to

9 N leads to an increase of hardness penetration depth by mm. Moreover, the reduction of temperature from RT togenic conditions results in an increase of the hardnesstration depth by 0.55 mm. This observation gives proof to

energy-based approach of cryogenic deep rolling. On the basise considerations on the induced DG in Fig. 1, an increase of thehanical load by the same amount should cause a constant DG

pendent from temperature. Reducing the temperature by atant value should on the other hand induce a certain DG

pendently from the mechanical loads applied. Fig. 9 presents amatic diagram showing how mechanically and thermallyced differences of Gibbs free energy superimpose in cryogenic

rolling. Reaching values between DGMs (martensite start) and

f (martensite finish) allows for surface hardening bytensitic transformation.

5. Outlook

Cryogenic deep rolling paves the way for industrial applicationof the advanced production line of cold surface hardening.Mechanically induced martensitic transformation of metastableaustenites allows for integration of the hardening step into themachine tool and thus carries the potential of economic savingsand high flexibility of the production. In future research projects,the transfer of the findings to industrial practice might besupported by (i) implementation of numerical models (ii)development of specific steel alloys with lower content of carbonand chrome (iii) investigation of the expected advantageousproperties of components in fatigue life tests.

Acknowledgments

The author greatly acknowledges funding of the project BR 825/61-1 by the German Research Foundation (DFG).

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mechani cal eff ect

of de ep rollin g

(Hertzian str ess)

depth below the sur face z

ind

uce

dd

iffe

ren

ce

offr

ee

Gib

bs e

ne

rgyΔ

G

the rmal eff ect of

CO2-sno w peenin g

the rmomech anical eff ect

(cryogeni c dee p roll ing)ΔGMf

ΔGMs

operational area for

martensi tic tran sformation

0.3-0.5 mm

Fig. 9. Superposition of mechanically and thermally induced difference of Gibbs free

energy in cryogenic deep rolling (schematic).

. Comparison of the hardness depth profiles after (cryogenic) deep rolling of

stable austenites with varied stability.

m

1.2

1.0

1.6

Effect of the cryogen ic

trea tmen t ΔRhttherm

Effect of increased

mechanical loa dsΔRhtmech under cryo-genic cond itions

TA = 1145 °C

deep rolled (RT)

0.4

0.6

0.8

0 1000 2000 3000 4000 N 600 0

rolling force Fr

Effect of increas ed mechanical load s

ΔRhtmech at RT

cryo. de ep rol led

. Dependency of the hardness penetration depth after deep rolling on the

g force and thermal conditions (TA = 1145 8C).

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