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colder treatment was included in this study. Therefore, the sampleswere cooled to 269 C for 168 h in liquid helium.

To ensure that the results were comparable to a previous studyof the wear of thermochemically treated steels [4], the same testapparatus was used and the same test conditions applied.

Studies of the effects of deep cold treatment on carburised andbearing steels have shown that it improves dry wear properties

the pin-disk machine shown in Fig. 2. The heat-treated test pieceis attached to the vertical bar as shown, and remains stationarywhile the disk rotates. The resultant pressure is high initially,equivalent to approximately 4 N/mm2 after 50 m sliding.

The end of the large bar has a small angle against its length.When the test pin is xed in a hole at the end of this bar the wearmark will not be in the centre, but at a circle between centre andthe outer diameter as illustrated in Fig. 3. Between each test, thetest pin is turned a little and xed again. This makes it possibleto do 612 tests with each test pin. The wear mark diameter canbe read with a microscope with a resolution of 0.01 mm

* Corresponding author. Tel.: +44 1484328736.

Cryogenics 49 (2009) 346349

Contents lists availab

ge

seE-mail address: paul.stratton@boc.com (P. Stratton).carburise and cold treat deliberately as they consider this toimprove wear. This type of treatment at 70 C was thereforeincluded in this study.

The benets of extended deep cold treatment on tool steelshave been widely reported [2]. Although some work on the effectsof deep cold on carburising steels has also been done, it did notexamine the effect on wear [3]. A typical deep cold treatment cyclewas therefore applied to case carburised samples to see if the sameeffect occurred as in tool steels. It has been shown that the longer asteel is held at the deep cold treatment temperature typically196 C in liquid nitrogen the better the result, so a longer and

Unterschleibheim, Germany in an Ipsen TR25 sealed quenchfurnace.

There were three carburising treatments followed by one offour post-carburising treatments (Table 2). Care was taken toensure that the cold treatment followed the quench within 1 h toprevent austenite stabilisation.

3. Wear testing

Wear testing was carried out at Swerea IVF AB in Sweden, on1. Introduction

Cold treatment for transformingof carburising steels has been a codecades [1]. Cooling the steel below tconverts any retained austenite to mSuch treatments are generally considimprecise control of the carburisinmuch carbon in the case. However,0011-2275/$ - see front matter 2009 Elsevier Ltd. Adoi:10.1016/j.cryogenics.2009.03.007d austenite in the caseindustrial practice fortemperature effectivelyte, increasing hardness.remedial measure afterss has resulted in toomanufactures do over-

signicantly [57]. This study set out to determine the extent ofthis improvement and the effect of colder and longer treatmentcycles.

2. Experimental

The samples (Fig. 1) were manufactured from a typical carburis-ing steel, 20MnCr5 with the analysis shown in Table 1. The heattreatment cycles were carried out at the Linde Gas laboratory inF. TribologyThe effect of deep cold induced nano-carhardened components

Paul Stratton a,*, Michael Graf b

a The Linde Group, Rother Valley Way, Holbrook, Shefeld S20 3RP, UKb The Linde Group, Carl-von-Linde-Str. 25, 85716 Unterschleissheim, Germany

a r t i c l e i n f o

Article history:Received 21 November 2008Received in revised form 13 March 2009Accepted 13 March 2009

Keywords:A. MetalsB. Nitrogen

a b s t r a c t

Most studies of the wear iment have been carried ouusing typical industrial cycwas found that under thincreased wear. However, tnicantly improve wear. Deven treatment for a much

Cryo

journal homepage: www.elll rights reserved.des on the wear of case

ovements produced by the nano-carbides formed during deep cold treat-tool steels. In this study a carburising steel 20MnCr5 was carburisedsubjected to a range of cold treatments and its wear performance tested. Itonditions examined, converting any retained austenite to martensitenano-carbides formed by extended deep cold treatment at 196 C did sig-er cold treatment at 269 C did not produce any further improvement,ger time.

2009 Elsevier Ltd. All rights reserved.

le at ScienceDirect

nics

vier .com/locate /cryogenics

ogenics 49 (2009) 346349 347P. Stratton, M. Graf / CryThe large horizontal grey beam in Fig. 2 can be xed in differentpositions so that a new track on the horizontal wheel at a differentradius can be used for each test. In these tests only one disk wasused so that all the results were comparable. The disk was madefrom a cold working tool steel hardened and tempered to 58RC.Its rotation speed was kept at the same sliding speed of 0.4 m/sfor all the tests. After a xed wear distance the large vertical barwith the pin xed at its end was put under a microscope to mea-sure the diameter of the wear mark. The sample carrier was thenreplaced in the exactly same position and the test continued.Although the diameter of the wear mark (Fig. 4) can be used asthe measure of wear, this investigation found that the calculated

Table 1Sample analysis.

%C %Si %Mn %P %S %Cr

0.195 0.40 1.25 0.35 0.35 1.10

Table 2The heat and cold treatments.

Treatment Conditions

Carburising treatments1 Carburised to 0.8% carbon and a total case depth of 0.75 mm and direct

oil quenched from 850 C2 Carburised to 1.0% carbon and a total case depth of 0.75 mm and direct

oil quenched from 850 C3 Carburised to 1.0% carbon and a total case depth of 0.75 mm, cooled

out, reheated, and oil quenched from 850 C

Cold treatmentsa Tempered at 150 C for 1 hb Cooled to 70 C for 1 h then tempered at 150 C for 1 hc Cooled to 196 C for 24 h then tempered at 150 C for 1 hd Cooled to 269 C for 168 h then tempered at 150 C for 1 h

Fig. 2. The pin-disk machine used for the wear testing.

Fig. 3. Wear marks at the end surface of the test pin.

Fig. 1. The wear test samples.Fig. 4. The test pin with one large and one small wear mark.wear volume was more discriminating. Each sample was subjectedto three tests and the results averaged.

4. Metallurgical examination

Separate samples of the same dimensions as the wear testpieces were treated in each batch. Sections were prepared andexamined for hardness proles and microstructure. Near surfaceretained austenite was determined by X-ray diffraction.

5. Results

5.1. Metallurgical

The hardness gradients for the case after carburising treatment1 and various cold treatments are shown in Fig. 5. It can be seenthat all the cold treatments increased the hardness near the surface

Fig. 5. The case proles for carburising treatment 1 after various cold treatments.

by a small amount because of conversion of retained austenite tomartensite. The total case depth was 0.65 mm and the effectivecase depth (to 550 HV) was 0.45 mm. The hardness gradients forcarburising treatment 2 and 3 samples were very similar withthe same total and effective case depths.

The retained austenite in the case of the samples after carburis-ing and cold treatment is shown in Table 3.

The microstructures of the carburising treatment 2 as-quenchedand tempered and after the various cold treatments are shown inFig. 6. The reduction in retained austenite produced by the coldtreatments can clearly be seen.

As may have been expected the microstructure of the near-sur-face case of carburising treatment 2 exhibits some ne globularcarbides (Fig. 7) produced by the cool-out, reheat, and quenchtreatment. The retained austenite was correspondingly lower thanwas found in the direct quenched treatment (carburising treat-

Table 3Retained austenite percentage for all the treatments at 50 lm.

Carburisingtreatment 1

Carburisingtreatment 2

Carburisingtreatment 3

Tempered 9.1 31.0 6.470 C, tempered 3.3 13.5 4.1196 C, tempered 3.0 9.6 4.0269 C, tempered 2.6 8.3 2.9

Fig. 7. The microstructure of carburising treatment 2 after reheat, quench andtemper.

Fig. 8. The wear of 20MnCr5 carburised to 0.8% carbon and direct quenched andsubjected to various cold treatments.

348 P. Stratton, M. Graf / Cryogenics 49 (2009) 346349ment 2).

5.2. Wear

The results of the wear tests are shown in Figs. 810. For thetwo treatments where there was little retained austenite present(Figs. 8 and 10) the changes in wear rate due to the cryogenic treat-ments were small, but signicant, with rates comparable to thosefound in a previous study of carburised steel using the same wearmeasurement technique [6]. As might have been expected theFig. 6. The microstructure of treatment 2 after quench and temper (a) and after various cold treatments (b, c, and d {see Table 2}).

It is highly likely that the mechanism for the improvement in wearis the precipitation of g-carbides, as has been shown to be the casefor tool steels [7,8]. Compared to the untreated sample, the overallwear rate was reduced by approximately 20% by deep cold treat-

P. Stratton, M. Graf / Cryogenics 49 (2009) 346349 349Fig. 9. The wear of 20MnCr5 carburised to 1.0% carbon and direct quenched andsubjected to various cold treatments.differences between the wear rates of the different cryogenic treat-ments were much larger when the initial retained austenite levelwas high (Fig. 9).

6. Discussion

It is quite clear from the wear test results that regardless of thecarburising treatment, cooling below 196 C, to 269 C, is of nobenet. This is probably because at 269 C the dislocations do notcoalesce to form nucleation sites for the nano-sized g-carbides,that are the basis of the wear improvements in the 196 C treat-ment [3,7,8], at these very low temperatures; diffusion is so slowthat even a 168 h treatment is insufcient. Even at 196 C theprocess takes a minimum of 24 h.

Both when the surface carbon is close to optimum (0.8%C(Fig. 5)) and when the high surface carbon is mitigated by formingcarbides that leave the matrix with the optimum carbon (1.0% car-bon, cooled out, then reheated, and quenched (Fig. 7)), the wearrate is minimised by the deep cold treatment at 196 C for 24 h.

Fig. 10. The wear of 20MnCr5 carburised to 1.0% carbon, reheated, and quenched,then subjected to various cold treatments.ment at 196 C for 24 h. In the 1.0% carbon sample that was directquenched, the effect of transforming the retained austenite to mar-tensite swamped any effect due to deep cold treatment.

It might be expected that transforming the retained austenite inthe high carbon, direct quenched samples (Fig. 8) would reduce thewear rate, but in fact the reverse was found, with the as-quenchedand tempered sample giving the lowest wear rate of all. It is prob-able that stress during wear dynamically transforms the retainedaustenite, minimising the wear rate. For most components it willstill be necessary to remove the retained austenite because of theproblems associated with grinding crack formation [9].

7. Further work

Although not considered in this study, it is recognised that theimprovement of resistance to sliding wear, due to cold or cryogenictreatments, is more or less signicant as a function of the slidingspeed [10]. Further work is required to elucidate this effect forthe materials and treatments studied.

8. Conclusions

The dry wear rate of optimally carburised 20MnCr5 was reducedby approximately 20% by deep cold treatment at 196 C for24 h.

Deep cold treatment at very low temperatures (269 C in liquidhelium) has no benet for carburised steel.

When high levels of retained austenite are present in the case ofcarburised 20MnCr5, cold treatment converts most retainedaustenite to martensite, consequently the dry wear rateincreases.

References

[1] Moore C. The development of the BOC Ellenite process (cold treatment ofmetals with liquid nitrogen. In: Heat treatment 73. The Metals Society; 1975.p. 15761 [book no. 163].

[2] Cold and cryogenic treating of steel. Heat Treat Prog; One-min Mentor2007;2(48).

[3] Stratton PF. Optimising nano-carbide precipitation in tool steels. Mater Sci EngA 2007;449451:80912.

[4] Surberg CH, Stratton P, Lingenhle K. The effect of deep cold treatment on twocase hardening steels. Acta Metall Sin 2008;21(1):17.

[5] Stojko A. Sub-zero treatment of tool steels. M.Sc. Thesis, DTU, Lyngby; 2001.[6] Stratton PF, Segerberg S. A comparative study of the dry adhesive wear of

various thermochemical surface treatments. In: Proceedings of internationalheat treat 2004. India: ASM; 2004. paper I-3.

[7] Gerson A, Cavallaro G, Xu N. Microstructure of cryogenically treated highperformance tool steels. Mater Aust 2007;40(5):489.

[8] Rhyim Y-M, Han S-H, Na Y-S, Lee J-H. Effect of deep cold cryogenic treatmenton carbide precipitation and mechanical properties of tool steels. Solid StatePhenom 2006;118:914.

[9] Totten GE. Steel heat treatment: metallurgy and technologies. CRC Press; 2006.p. 387.

[10] Bensely A, Prabhakaran A, Mohan Lal D, Nagarajan G. Enhancing the wearresistance of case carburized steel (En 353) by cryogenic treatment. Cryogenics2005;45(12):74754.

The effect of deep cold induced nano-carbides on the wear of case hardened componentsIntroductionExperimentalWear testingMetallurgical examinationResultsMetallurgicalWear

DiscussionFurther workConclusionsReferences