The Effects of Cryogenic Tempering on Tool Steels
Introduction
The ability to work metals, with the possible exception of writing and the printing press, is the greatest invention of mankind. This technology has been used to define the ages of civilization. Even today we refer to primitive cultures as “stone age.” The discovery of “The Iceman” in the early 1990s pushed our definition of the copper age to a time before 3500 B.C. and helped us understand that the early copper tool makers were probably the first tradesmen. These artisans spent much time improving their craft and were able to obtain a
higher standard of living by focusing on this unique skill and trading the copper tools they made for an abundance of other necessities.
Figure 1. The Iceman’s axe.
The ability to work metal created the first true economy. The metal craftsman found that they could have a richer life by specializing in metal working and trading the metal products for an abundancy of other goods.
It surprises many people that it took metal workers more than 2500 additional years to make iron products common. The reason for this delay rests on two properties of iron. One is that it melts at 1538ºC versus copper’s much lower 1084.9ºC and the other being that carbon is needed to make it strong. Pure iron is actually quite soft.
The fact that iron/steel was even discovered at all by the ancients rested on two interesting flukes of nature. The first being that iron ore (mostly iron oxide) can be “reduced” to iron by being exposed to hot carbon. The second is that iron is changed to steel with the addition of small amounts (< several percent) of carbon. These metallurgical facts, added to the ubiquity of carbon in a typical ancient fire, resulted in small pieces of iron/steel being discovered in the copper smelting processes when naturally occurring iron ore was accidentally placed in the fire. The need for high temperature to melt the iron meant that only small amounts of iron were produced. Metallurgists had to wait until the 18th century before iron could be cast in large volumes. Proof of this inability can be seen in the many brass cannons that were used until about 1850.
Unfortunately, the ancients did not know the role of carbon content in the iron and for centuries were frustrated in their inability to repeat a process to produce steel. It was not until the time of the Romans that steel production was consistent; their ability to produce steel, in no small way, resulted in their conquering most of the known world.
From Roman victories to those of Napoleon, Eisenhower and Schwarzkopf, steel was the foundation of the victorious societies. The wide variety of properties that steel can acquire when alloyed, its relatively low cost and its ability to be formed in many different shapes have assured that steel was and will be the most important material of humankind.
Until 1968 more technical papers were written about steel than any other material. Given its long history, it is still the most studied material on earth.
Cryogenic Tempering of Tool Steel
Considering the age and wide uses of steel, it is surprising to many people that there is still much we do not know about it. One of the areas related to steel that we are still acquiring knowledge about is cryogenic tempering. This paper will review what is know about cryogenic tempering and future research plans of C.I.N.E. to improve our understanding of this important phenomena.
Cryogenic tempering takes place in a chamber, where the materials are gradually lowered in temperature. Shallow cryogenic tempering is performed at about -120ºF (-85ºC) for 10 hours or so, whereas deep cryogenic tempering takes the material below -300ºF (-185ºC) for more than 24 hours. The materials are then slowly raised to room temperature and usually annealed at about 300ºF (149ºC) for several hours. Only deep cryogenic tempering has shown to give the greatest improvement in wear resistance.
Controlled experiments and industry experience have demonstrated that many materials benefit from this treatment. Increased wear life and better corrosion resistance, while at the same time maintaining or even improving toughness have been observed. However, few materials benefit more than tool steels. Tool steels that are deep cryogenically treated will typically last more than 50 % longer than as quenched specimens. In addition, tool steels have been studied extensively to understand the cryogenic phenomena. Therefore, I will discuss the present understanding of the cryogenic tempering in these metals.
Martensite Formation
Even the ancients knew that the rapid quenching of steel from elevated temperatures made it harder. We now know that this rapid quenching produces a metastable phase in steel called martensite. This transformation process is rapid and diffusionless (i.e. no long distance motion of molecules is required.) To form martensite the steel must initially be in the face center cubic (FCC) form of iron called austenite (abbreviated: g). To establish austenite in the steel, one typically has to “soak” the steel at temperatures above 750ºC. There is another form of iron called ferrite (abbreviated: a). If the steel is ferritic, it cannot form martensite.
Figure 2. The steel phase diagram.
From the above phase diagram, plain carbon steel with 0.6% carbon must be heated and “soaked” at about 760ºC to produce austenite. Rapid cooling from this temperature will produce martensite. If the steel was only heated to 700ºC ferrite and iron carbide would be the starting structure. Marteniste cannot be produced from these phases.
The martensite reaction occurs as the FCC austenite transforms to a BCT (body centered tetragonal) martensite. Carbon atoms are trapped in interstitial sites during the rapid transformation. See Figure 3. As the carbon content increases a greater number of carbon atoms are trapped. If the temperature quench is not rapid, stable pearlite, bainite and other constituents can form.
Figure 3. Martensite crystal structure.
As the steel is quenched, the martensite starts to form at a given temperature for the material. This temperature is called the martensite start temperature (Ms). To completely form martensite, the martensite finish temperature must be reached before pearlite and cementite can start to form. This mechanism can be seen on a time-temperature-transformation chart (TTT chart) shown below.
Figure 4. TTT Diagrams for (a) 1050 and (b) 10110 steels
The addition of carbon reduces the temperature at which martensite stops forming as we can see in the above TTT charts. The Mf temperature is the temperature at which 100 percent of the martensite has formed. Mf for 1050 steel (0.5% carbon) is 245ºC, whereas it is 85ºC for 10110 steel (1.1% carbon). The presence of other alloying materials such as manganese, chromium and vanadium tends to reduce the Mf temperature and provides the added benefit of suppressing the formation of pearlite and bainite. The combination of the carbon and the alloying metals can reduce Mf to below room temperature, with no possibility of other constituents forming. The TTT below, for A2, shows this effect.
Figure 5. IT diagram for A2 tool steel
The discussion leads us to the first mechanism of cryogenic tempering: finishing the martensite formation process if Mf is below the final quenching temperature. This mechanism would most likely be found in tool steels as their high carbon and alloy content makes the reduction of the Mf temperature below typical quench temperatures common. However, it would be surprising if this mechanism required temperatures below -150ºF.
The change in metal structure after cryogenic tempering of an S7 tool steel is shown in Figure 6.
Figure 6. Before and after micrographs of an S7 tool steel.
The increased martensitic structure is evident in the after micrograph on the right. Original magnification was 1000x.
The Role of Eta (h) Carbides
Meng etal1 proposed that the dominant mechanism for enhancing wear in tool steels is eta carbide formation.
Their work is experimentally well supported and the analysis of eta carbides was performed with transmission
electron microscopy. Their conclusions, from experiments, were:
Figure 8. Cryogenic tempering appears to cause a movement of carbon atoms in the steel molecular structure as
shown in the diagram. The results are evident in the before and after x-ray diffraction pattern.
1. Finishing the martensite formation process results in a barely measurable improvement in wear resistance.
2. The martensite formation process can be finished with shallow cryogenic treatment (i.e. about -100ºF.)
3. The dominant method of wear reduction is the formation of eta carbides.
Eta carbides actually form a crystal structure intermeshed with the martensite. They can only be seen at the types of magnifications produced in electron microscopy (e.g. > 20,000X) or by using x-ray diffraction. Their formation involves a sub-atomic movement, which is a non-diffusional process understandable at low temperatures. Eta carbides are molecular in size. Figure 8, shows both the eta carbide structure and how it forms and x-ray diffraction patterns before and after eta carbide formation.
Figure 9. Wear Rate vs Sliding Speed. Note that the deep cryo tempered steel has < 1/7th the wear rate of the untreated steel.
Carbide Particle Formation (Fe3C)
Part of the cryogenic process typically involves tempering after the cold processing. Kramer2 has proposed that: For higher carbon steels the cryogenic effect is more understandable given the dependence of Ms and Mf temperatures
on carbon concentration and potential effects of nucleation of Fe3C on final tempered particle size.
When marteniste is tempered a mixture of ferrite and cementite (Fe3C) is formed. Kramer is suggesting that the cryogenic and tempering processes produce a grain structure of these two phases that has greater wear resistance. It is almost like the hard cementite is held like sand in sand paper in a matrix of ferrite. This would create a tough yet wear resistance material. The cementite particles would be on the order of 0.1-1 micron in size.
Stress Relief
It is believed that the process of deep cryogenic treatment and tempering provides relief of internal stresses in the metal.
This phenomenon should result in better corrosion resistance. At C.I.N.E. we plan to partner with Worcester Polytechnic
Institute to investigate this phenomenon using x-ray diffraction and other techniques.
Examining Test Data in Light of These Proposed Mechanisms
Typical experimental data1,3-5 show these effects in tools steels:
1. Rockwell C hardness increases modestly: 1-3 points starting at 58-62
2. Wear resistance increases significantly: 50 to greater than 500% improvement
3. Corrosion resistance increased
The addition of martensite will increase the hardness and increase wear resistance, but reduce the toughness. This small increase in hardness and yet dramatic increase in wear resistance would not point to increased martensite formation as the most significant mechanism.
The formation of the molecular eta carbides and fine cementite particles in the final temper structure is most consistent in interpreting the first two experimental observations. Eta-carbide is known to be strong and tough1 and the fine cementite particles should increase wear resistance without sacrificing toughness.
Corrosion is known to increase in the presence of stress, the stress relief mechanism would appear to likely explain this observed benefit.
Conclusions
The dramatic improvement in wear resistance in deep cryogenicly treated tools steels, with no loss in toughness is
most likely explained by the formation of molecular eta carbides and the formation of fine cementite particles in the
final tempered structure. It would appear that the conversion of additional martensite, although often present, is
probably a secondary mechanism. This understanding also supports the increase wear resistance in materials that don’t
readily form martensite.
It is our intent at C.I.N.E. to further elucidate the mechanisms of cryogenic tempering to improve and expand the use of this valuable process.
References:
1. Meng, Fanju, etal, Role of Eta-Carbide Precipitations in the Wear Resistance Improvements of
Fe-12Cr-MoV-1.4C Tool Steel by Cryogenic Treatment, ISIJ International, Vol 34 (1994) No. 20 pp 205-210.
2. Kramer, E. K,, private communication April 2002.
3. To be added.
By: Ronald C. Lasky, Ph.D., PE