1. Effect of Cryogenic Treatment on Steels Manjunath M Project Intern UVCE Bangalore university

2. INTRODUCTION Cryogenic Treatment(CT) is the process of cold treatment below the sub-zero temperature. It is an inexpensive permanent treatment affecting the entire section of the component unlike coating. Cryogenic treatment Improves hardness, fatigue resistance, toughness, and wear resistance of steel.

3. What does Cryogenic Treatment Do to Steel? It is increasing Wear Resistance and Toughness of steel by interplay of three effects.  Completing Martensitic Transformation  Promoting Uniform precipitation of fine Carbides  Relieve Residual Stresses

4. The third law of thermodynamics states that entropy is zero at absolute zero temperature. Deep sub-zero treatment uses this principle to relieve stresses in the material This leads to ironing out of the defects in the material and also attainment of the minimum entropy state. Grain shape and size gets refined and homogeneous. Defect is elimination

5. Process Of Cryogenic Heat Treatment Heat treatment sequence for maximum martensite transformations Quenching Cryogenic processing Tempering Austenitize

6. Austenitizing Austenitization which means to heat the iron, iron based metal or steel to a temperature at which it changes crystal structure from ferrite to austenite. During austenitizing, the final alloying element partitioning occurs between the austenite matrix (that transforms to martensite on cooling) and the retained carbides.

7. Quenching After Austenitizing temperature, the steel is cooled to ambient temperature rapidly in a suitable quenching media like water, oil, air. Once the austenite is cooled below its critical temperature, it becomes unstable and it starts to transform into martensite. After conventional heat treatment(CHT) there would always be some retained austenite in the steel, which is up to 20-30%.

8. Cryogenic Processing Cryogenic Treatment (CT) of tool materials cooling temperature consists of two stages. 1) Shallow Cryogenic Treatment (SCT) 2) Deep Cryogenic Treatment (DCT) RAMP DOWN SOAKING RAMP UP TEMPER HOLD

9. Cryogenic process of DCT and SCT

10. Block diagram of the cryogenic equipment

11. Tempering Tempering consists of heating a hardened steel to a temperature below eutectoid temperature. This makes it softer and ductile. The carbon trapped in the martensite transformation can be released by heating the steel below the 723˚C transformation temperature. This release of carbon from nucleated areas allows the structure to deform plastically and relive some of its internal stresses

12. Tempering reduces residual stress, increases ductility, toughness and dimensional stability. Slightly higher hardness compared to conventional heat treatment. The degree of tempering is dependant on temperature and time; temperature having greatest influence on wear rate.

13. Phase transform on Cryogenic Treatment

14. FCC Austenite Possible positions of Carbon atoms Only a fraction of the sites occupied C along the c-axis obstructs the contraction Possible positions of Carbon atoms Only a fraction of the sites occupied Tetragonal Martensite FCC Austenite Alternate choice of Cell 20% contraction of c-axis 12% expansion of a-axis

15. Phases of Steel and their Characteristics

16. Retained Austenite Martensite Tempered Martensite Microstructure Martensite needles Austenite 60  m

17. Properties of cryogenic treatment In sub-zero temperature D2 steel, increasing austenitizing temperature increases amount of retained austenite. After CT presence of retained austenite(γ) in both SCT and DCT is 4-5% and less than 1% respectively. In DCT, reduction in austenite content will increase the amount of secondary carbides by ≈47% and 38% in comparison to CHT and CT, respectively.

18. During CT contraction in lattice martensite and austenite took place. Due to super saturation martensite with carbon and thermodynamic instability the carbon atoms squeezed out of martensite, migrated to the neighbouring lattice defects and acted as nucleation sites for the growth of fine carbides. Increase of population density of secondary carbides and size refinement. Precipitation of fine carbides along grain boundary.

19. SEM micrographs of AISI D2 steel (a) CHT, (b) SCT and (c) DCT specimens

20. Cryogenic treated steel samples increases hardness due to the conversion of retained austenite to martensite and due to the presence of fine carbides in the metal matrix. CHTSCTDCT HRC6464.565.5 Vickers Hardness 920934980 AISI M35 steel

21. Hardness of the D2 steel increases maximum up to 36hrs of cryogenic processing and decreases further increase in cryogenic processing time. But there is no much effect on the hardness.

22. Flank Wear: Cryogenic treatment yields 110.2%, 86.6% & 48% improvement in T1, M2 & D3 steels respectively as compared to standard heat treated(SHT) tool samples. Sliding wear: As the Load increases @ higher velocity in AISI D2 steel there an improvement in wear resistance due to DCT compared with cold treatment. AISI D2 steel

23. Fine precipitates of carbides formed during deep cryogenic treatment increased the wear resistance. Because of the presence of fine carbides the plastic deformation had not occurred. SEM images of AISI M35 CHT and DCT pin disc wear tester

24. In DCT, increases hardness and abrasive resistance due to abundant retained austenite transforming into martensite and secondary carbides precipitation. In high speed steel, Wear rate increases as the tempering temperature increases after the cryogenic treatment. So tempering temperature should be low for the wear resistance.

25. During subsequent double tempering at 200 ̊C after cryotreatment, So the internal stresses were relieved and the martensite was conditioned with a consequent improvement in toughness. Therefore, there was no loss of toughness due to cryogenic treatment. AISI M35 steel

26. Fracture toughness of AISI D2 steel that the ductility of tempered martensite matrix is controlled by the amounts of dissolved carbon and alloying elements. AISI M2 steel, three Tempering cycle DCT With holding time of 2hrs Each tempering achieve Homogeneous metallur- -gical Property.

27. The effect of retained austenite on the properties of steel includes decrease in tensile strength, fatigue resistance, yield strength, and compressive residual stresses. DCT improves the same by transformation of retained austenite to martensite. The effect of SCT on T1 type-high speed material attain the maximum hardness of 67 HRC. Even if the deep cryogenic treatment does not influence the hardness, it increases both toughness and wear resistance

28. Maximum compressive residual stresses develop after DCT, which are beneficial in maintaining an appreciable endurance limit. DCT steel when subjected to tempering has undergone a reduction in compressive residual stress. Such stress relieving behaviour was mainly due to the increased precipitation of fine carbides in specimens subjected to DCT with tempering. Progressive reduction in compressive stress CHT(245MPa) to SCT(145MPa) and DCT(115MPa)

29. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Effect of tempering temperature on the properties of eutectoid steel.

30. Cryogenic Effect on Different Steels AISI 4340 - Hardness and Fatigue strength is higher for CT whereas toughness was lower when compared to that of the conventionally treated steel. Carburized Steels - 18NiCrMo5 are hardness and tensile tests by optical fractographic. Hardness increases (+0.6 HRC to +2.4 HRC) for all CT groups and tensile strength (+11%) in one case. D2 tool Steel – Austenitizing (1283-1373K) DCT 77K and single tempered 473K had maximum hardness, greater toughness for lower austenitizing temperature and wear rate is lowered compare CHT.

31. T1, M2 and D3 steel – CT 110% improve tool life and yields 3%,10% and 10.6% extra tool life over tempered CT. M1, EN19 and H13 tool steels - Improvement in wear resistance after cryogenic treatment (0, -20, -40, -80, -190˚C). High-Speed Steels – DCT at -196˚C double the service life, increases hardness and toughness simultaneously. DCT improves microstructure by producing fine martensitic structure. Casting – high-chromium cast iron DCT improves hardness and abrasion WR by reduction of retained austinite & precipitating fine secondary carbides.

32. Percentage Increase in Wear Resistance after Cold Treatment and Cryotreatment

33. Different steels AISI M1,M2,T1,D2,D3,H13,M35, S2 tool steel M3/2 HSS matrix with Nb and Ta carbides Fe-12Cr-Mo-V-1.4C tool steel En19,En353 carburized steel Carburized steel AISI 4140 cold rolled steel and 4340 low alloy steel 13Cr2Mn2V high Cr white iron & 3Cr13Mo1V1.5 high Cr cast iron

34. Applications Cutting tool for different machining operations: Sawing, milling, drilling, broaching, turning, slitting, shearing. Metal forming tools: dies, molds, punches. High precision parts: gauges, guides, shafts.

36. Benefits of Cryogenic treatment Homogenizes the Crystal Structure Improved structural compactness Prevents concentrated Heat Built-up Increase resistance to abrasive wear Changes the entire grain structure of the metal, not just the surfaces Refinishing or regrinds do not affect permanent improvements Eliminates thermal shock through a dry, computer controlled process Transforms most retained austenite to hard martensite Forms micro-fine carbide fillers to enhance carbide structures

37. Increases durability and wear life Decreases residual stresses in tool steels Decreases brittleness Increases tensile strength, toughness and stability Works on new or used tools Reduced down time, less maintenance and higher productivity Deep cryo-processing is compatible with other treatments (TiN, Chrome, Teflon etc.) High alloy steel cutting tools stay sharper longer, fewer micro-cracks, less chipping Results in the orderly arrangement of crystals, increases internal bonding energy, and achieves a structural balance throughout the mass of the material

38. Grain structure changes in Cryo-treated materials 50%-75% decrease in grain size in treated specimens Much more uniform distribution of carbon Alleviation of grain boundaries

39. Caution on Cryogenic Treatment Soaking period is 12-72hrs, more than 72hrs no effect on material. On AISI M2 high speed steel, multiple tempered after DCT and more than 3 times tempering decreases the hardness.

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