The technology of CT was developed in the 1960s and still elicits contrary scientific opinions today. Some studies report that CT improves hardness, wear resistance, bending strength, toughness, fatigue strength, etc., but some scientists do not agree. Also, experts do not agree as to the main factor influencing results when CT is applied – austenitizing temperature, cooling rate, quench temperature, holding time, heating rate or tempering temperature.
The mechanism of fine carbide precipitation provides the most serious discussion. When exactly do they form, and what is their effect on mechanical properties? The steel grade treated by CT is also important. A significant effect is seen for X153CrVMo12 (AISI D2) steel, and others are more disputable. Our experimental work is focused on PM-produced, wear-resistant ledeburitic tool steels. The goal is to determine the optimal CT (temperature and time) that can help to prolong the life cycle of cold-work tools, keeping in mind the economics of additional treatments. The investigation discovered many interesting results but raised even more questions.
Introduction
Ledeburitic steels contain ledeburite in their structure – typically present when the carbon content is 2.06-6.67% – despite quite low carbon content. It is possible thanks to alloys like Cr, V, W and Mo, which extend the ferrite area and close the austenite area. Alloying moves the eutectoid point (S) and the carbon solubility in austenite (E) toward lower values of carbon content.
The structure of quenched ledeburitic steels consists of martensite, retained austenite and undissolved carbides of primary, eutectic and partially secondary origin. The amount of retained austenite increases with higher austenitizing temperature due to a greater saturation of austenite with carbon and alloying additions that lower the martensitic-start (MS) temperature.[1] Moreover, exceeding the optimal austenitizing temperature means that MS falls well under subzero. For example, the MS temperature of X153CrVMo12 (1.2379, AISI D2) is 175˚C (347˚F) for an austenitizing temperature of 1050˚C (1922˚F) but -100˚C (-148˚F) for an austenitizing temperature of 1200˚C (2192˚F).[1]
To reach a higher level of martensitic transformation and reduction of retained austenite, it is necessary to insert a CT between quenching and tempering (Fig. 1).
CT is accompanied by changes of mechanical properties such as toughness, hardness, fatigue strength, fracture toughness and wear resistance. The impact of CT on mechanical properties is significant and positive depending on the steel grade. Most studies agree that the effect of CT on high-speed steels is more negative and does not bring an advantage. Another opinion dominates for Cr and Cr-V ledeburitic steels where wear resistance, in particular, increases significantly (Fig. 2).[2]It is necessary to follow the CT with several tempers. Tempering is a very important process in forming useful tool-steel properties because most have secondary-hardening capabilities. This is why all studies that deal with CT should also consider optimizing the tempering process.
Experiments
Previous Experiments – Uddeholm VANADIS 6 Steel
A team of practitioners and scientists was established to investigate how to prolong the life cycle of cold-working tool steel with requirements for high wear resistance and good toughness. Uddeholm VANADIS 6 was chosen as an advanced powder metallurgy (PM) Cr-Mo-V alloyed steel, which is characterized by very high abrasive/adhesive wear resistance and high toughness to prevent premature failure due to crack formation. The life cycle is dependent on heat treatment and, on the other hand, heat treatment depends on tool application. According to the producer material recommendation, hardening from the temperature of 1050˚C (1920˚F) and tempering three times at a minimum of 525˚C (980˚F) is the optimal recipe for high wear resistance of VANADIS 6.
Two types of specimens were made. The first, Ø17 x 10 mm, was used for a structural investigation and hardness measurement. The second, 10 x 10 x 100 mm, was used for three-point bend testing. These samples were fine ground to a surface roughness of 0.2-0.3 mm. Heat treatment was a combination of the following (Fig. 1):
• Vacuum austenitizing at 1000-1075˚C (1832-1967˚F) for 30 minutes
• Nitrogen gas quenching at 5-bar pressure
• CT in nitrogen or liquid nitrogen at -90˚C (-130˚F) for 4 or 10 hours and -196˚C (-320.8˚F) for 4 or 10 hours
• Double tempering at temperatures of 530 or 550˚C (986 or 1022˚F) for two hours
• One set of samples was treated without CT to compare results.
Structural differences were investigated by light metallography; hardness was measured; and three-point bending tests were performed with a distance between supports of 80 mm, central-region loading and rate of 1 mm/minute up to the moment of the fracture.[3]
Results
Conclusions from the VANADIS 6 investigations are:
• Three-point bending strength is generally higher for CT samples at -196˚C (-320.8˚F) for 4 hours than the same at -90˚C (-130˚F) for 4 hours or no CT. The significance of this difference increases with higher austenitizing temperature.
• Lengthening the subzero holding time to 10 hours does not bring any toughness benefits as determined by the three-point bending strength.
• The hardness of CT samples (-196˚C/-230.8˚F for 4 and 10 hours) is about 2.5 HRC lower than that of VANADIS 6 with no CT and is the same as after deep cooling to -90˚C (-130˚F) regardless of the subzero holding time.
• The microstructure of CT-processed samples is different from no subzero treatment in many ways. Nevertheless, it should be evaluated by TEM for a better description.[3]
Generally, there are no significant advantages that would prolong the lifetime of tools by providing higher wear resistance and toughness. We can perceive certain improvements, but CT does not make sense considering the economics. Real experiments with tools made from VANADIS 6 did not show a longer life cycle.
Other Tool-Steel Investigations
Uddeholm SVERKER 21 (AISI D2) and VANADIS 4 EXTRA
Cold-work testing of VANADIS 6 was done in cooperation with Uddeholm. They turned our focus to CT conventional tool steel Uddeholm SVERKER 21, which maintains its life cycle twice as long in comparison with VANADIS 6. Thanks to a discussion with Uddeholm specialists in the Czech Republic, our team decided to add Uddeholm VANADIS 4 EXTRA to our CT tests.
SVERKER 21 (AISI D2) is Cr-Mo-V-alloyed steel made by conventional metallurgy with high wear resistance, dimensional stability, high compression strength, good hardenability and good tempering resistance. VANADIS 4 EXTRA is Cr-Mo-V-alloyed PM steel with very good toughness, high abrasive and adhesive resistance, high compressive strength, good hardenability and good tempering resistance. The chemical composition of all investigated materials is shown in Fig. 3.
Specimens were made from both steels with dimensions 10 x 10 x 120 mm for three-point bending tests and for hardness measurement. The optimal heat treatment was chosen based on our previous experience with the VANADIS 6 investigation:
• Vacuum austenitizing at 1025˚C (1877˚F) for 30 minutes
• Nitrogen gas quenching at 5-bar pressure
• Cryogenic treatment in nitrogen or liquid nitrogen at -90˚C (-130˚F) for 4 or 10 hours and -196˚C (-320.8˚F) for 4 or 10 hours
• Tempering temperature was chosen in order to reach maximum hardness of secondary hardening – 480˚C (896˚F) for SVERKER 21 and 530˚C (986˚F) for VANADIS 4 EXTRA
• One set of samples was treated without CT to compare results.
Results
Three-point bending tests were performed on both material specimens (under the same conditions as VANADIS 6) and compared with the results obtained during that investigation. Figure 4 demonstrates bending strength measured for four regimes:
• Q = as-quenched and tempered
• 90/4 = as-quenched, CT at -90˚C (-130˚F) for 4 hours and tempered
• 90/10 = as-quenched, CT at -90˚C (-130˚F) for 10 hours and tempered
• 196/4 = as-quenched, CT at -196˚C (-320.8˚F) for 4 hours and tempered
• 196/10 = as quenched, CT at -196˚C (-320.8˚F) for 10 hours and tempered
Hardness was also investigated. Figure 5 shows the comparison of measured hardness for the same materials and heat-treatment regimes described in the three-point bending test.
Looking at the graphs, it is obvious that the bending strength of VANADIS 4 EXTRA generally exceeds the other two materials. For each material, the CT regime does not influence the three-point bending-strength values significantly. We can evaluate the differences among three types of steels only.
Comparison of hardness values proved the as-quenched and tempered samples as the hardest for both VANADIS grades. Regarding SVERKER 21, heat-treatment variations do not impact the hardness. The highest hardness for samples with no CT corresponds with the lowest three-point bending strength, demonstrating relations between hardness and toughness. We would expect higher hardness values due to the subzero treatment, but it is more complex than this because at least three phenomena occur during the tempering process:
1. Martensite tempering is always associated with a decrease in hardness. However, it is assumed that martensite developed after the CT process does not differ significantly from that developed after simple quenching, despite higher tetragonality.[2,3]
2. The retained austenite transformation during cooling from tempering temperature, which corresponds with the secondary-hardness peak, causes the increase in hardness. Logically, the contribution of retained austenite transformation to the increase in hardness is expected to be less if CT had been performed.
3. The precipitation of carbides during tempering is the next reason for hardness increase. Stratton, et.al. assumed that the precipitation of nano-carbides can occur as early as during the hold at the CT temperature.
Conclusions
1. Three-point bending strength is significantly higher for VANADIS 4 EXTRA, especially for -90˚C (-130˚C) for 4 hours. Differences among another CT options, including no CT, are not great. The lowest values were seen for SVERKER 21.
2. Longer CT time does not enhance three-point bending strength or hardness, which is the same for all investigated materials.
3. The hardness of CT materials is lower in comparison with no CT by 1-2 HRC and does not depend on the CT holding time.
4. The differences that have been proved so far between CT and no-CT materials are not too practically important considering CT costs.
5. Investigators are not unified in the opinion of CT benefits, although research has been going on for more than 50 years. General conclusions and achieved results do not differ much steel by steel.
6. Previous investigations show that we must pay more attention to the steel grade. The winner of the described investigation is VANADIS 4 EXTRA, which is powder-metallurgy produced. Outsider SVERKER 21, produced by conventional metallurgy and CT, has twice the life of VANADIS 6.
7. It is of value to continue research of wear-resistance tests, microstructure evaluation, retained austenite measurement, etc. The most important will be real testing of tools made from VANADIS 4 EXTRA.
Acknowledgements
Authors wish to thank the Ministry of Industry and Trade of the Czech Republic for the financial support for the solution of the project TIP FR-TI1/003. IH
For more information: Contact Petra Salabova or Otakar Prikner at PRIKNER – tepelne zpracovani kovu, s.r.o., 54973 Martinkovice 279, The Czech Republic; e-mail: p.salabova@prikner.cz; web: www.prikner.cz
References
1. Berns, H., HTM 29 (1974) 4, p. 236
2. Stratton, P.F., “Process Optimalization for Deep Cold Treatment of Tool Steels,” Proceedings 1st International Conference on Heat Treatment and Surface Engineering of Tools and Dies; Zagreb, Croatia, 2005, pp. 11-19
3. Sobotova, J. et al., “Structure and Properties of Subzero Processed VANADIS 6 PM Ledeburitic Steel,” METAL 2011 - 20th International Conference on Metallurgy and Materials; Brno, The Czech Republic, 2011
4. Jurci, P. et al., “Sub-zero Treatment of PM VANADIS 6 Ledeburitic Tool Steel,” 9th International Tooling Conference, Leoben, Austria, 2012
5. Oppenkowski, A. et al., “Evaluation of Factors Influencing Deep Cryogenic Treatment that Affect the Properties of Tool Steels,” Journal of Materials Processing Technology, 210, 2010
6. Chi, H.-X. et al., “Effect of Cryogenic Treatment on Properties of CrS-Type Cold Work Die Steel,” Journal of Iron and Steel Research, International, 2010, 17(6): 43-46,59
7. Badissera, P., Delprete, C., “Deep Cryogenic Treatment : A bibliographic Review,” The Open Mechanical Engineering Journal, 2008, 2
