Austenitizing Part 3 – Multi-Step Austenitizing

There are many modifications to a straight high temperature austenitize for a given hold time followed by quenching. I am covering a few of them in this article.

Preheating

Preheating is performed to minimize size change, distortion, and cracking during heat treatment. Often a single preheating is recommended, but for some grades two preheating temperatures are recommended. For example, the Vanadis 4 Extra datasheet recommends a first preheat temperature of 600-650°C and a second of 850-900°C, such as in the following schematic [1]:

During heating the steel grows following the coefficient of thermal expansion. Therefore, sections which are at higher temperature (such as the surface) have grown more than the core. Furthermore, austenite is more dense than ferrite so the sections of the steel that have transformed to austenite shrink. Between these factors complex stresses can be induced into the part. This is particularly true for thick sections and complex shapes. Rapid heating compounds this problem such as when using salt bath hardening. Steels with high austenitizing temperatures are also typically worse because the thermal gradients can be more extreme. Some alloying elements affect thermal conductivity, such as high chromium additions, so preheating is more necessary for stainless steels or high chromium tool steels like D2 [2].

Prequenching

With high speed steels it was discovered that double austenitization led to discontinuous grain growth for a combination of very large grains along with fine grains. These large grains are deleterious to toughness. Finer grains lead to an increase in both strength and toughness [3]:

Typically an increase in strength leads to a reduction in toughness, so grain refinement is highly desirable as it can increase both properties at the same time.

Many studies were performed in an attempt to solve the mystery of why multiple austenitizing cycles on high speed steels led to discontinuous grain growth unless given an annealing treatment in between. As part of this research, it was discovered that if the first austenitization was at a sufficiently low temperature, the grain size was actually refined [4]:

Notice that when the first austenitization temperature was between 1600-1900°F, the grain size was decreased from a intercept size of 14 or so up to 19-20. A similar effect was found by other researchers [5-6] and also on other steels, such as D2 [7]:

They reported that the fine carbides precipitated from the prior martensitic microstructure developed from the prequench were much finer than what would be found in an annealed microstructure. Therefore, there is a high density of fine carbides from the austenite to nucleate from, resulting in a high density of fine austenite grains. That can be illustrated by the following schematic showing the small temper carbides that act as nucleation sites for austenite:

Rapid Cycling for Grain Refinement

Grange reported grain refinement through cycling above and below the austenite transformation temperature back in the 60’s [3, 8-9], and similar methods have become popular among knifemakers. This method has been given many names, including double austenitization, triple quenching, multiple quenching, rapid cycling, etc. Grange studied several steels from low carbon, medium carbon, and high carbon steels including 52100 [3-5]. Grange found that in general, short hold times and lower temperatures (but above austenite transformation) were better for maintaining a fine grain size, and that the use of lead baths provided rapid heating to further limit grain growth. He also found that martensite was a better condition to austenitize from than ferrite-pearlite for achieving a finer grain size. Here is an example of grain refinement he achieved with 52100 [3]:

Grange concluded that a martensitic prior microstructure led to finer grains because the tempering of martensite during heating led to a fine distribution of carbides for many nucleation sites, leading to a high density of fine austenite grains, which is essentially the same as the theories given for prequenching. He does not cite the prequenching studies so it is unclear if he was aware of those articles.

Once the new finer austenite grains are formed and quenched back to martensite, further cycling can occur. The new finer grain size has an even higher density of nucleation sites as the prior austenite grain boundaries also serve as nucleation sites. Further cycles of austenitizing and quenching leads to yet finer grain size until a point is reached where the driving force for grain growth is so great that no further refinement is practically possible. Grange also reported that rapid cycling of high carbon steels like 52100 led to a very fine distribution of carbides, which could also improve properties. Grange reported [9] that the toughness of 1045 was increased significantly through rapid cycling:

Grange used heating in lead baths to achieve rapid heating which have relatively fast heating rates. Verhoeven [10] used salt baths with three austenitizing and quench treatments at 1450°F for 4 minutes to refine the grain size of 1045, 1086, and 52100 from an ASTM grain size of 8.5-11 to 14-15. Despite the rapid heating rates of lead baths and salt baths, slower heating methods can also be used. Kim et al. [11] compared a double austenitization treatment with a muffle furnace and a 1h hold to rapid cycling with induction and still found grain refinement with the furnace; however, the rapid induction was more effective. With a shorter hold time or perhaps lower temperature in the furnace the results could likely be somewhat closer.

Another type of prequench that has been used is an intentionally higher temperature in the first austenitize followed by a lower temperature austenitize. The intent of these treatment is not to refine the grain size but to dissolve more carbide to improve toughness. This was originally performed on a 0.4C-4Cr steel [12], but has since been performed on others [13-16]. On a study of 431 stainless steel [13], it was found that using 1070°C followed by 1000°C austenitizing cycles led to a grain size intermediate between a single austenitize at 1000°C and 1070°C, but had a lower carbide volume than a single 1000°C for an improvement in toughness.

Because an upper temper was also utilized which transformed most/all of the retained austenite, the effect of retained austenite can be separated from the impact of lower carbide volume. Accross the different tempering temperatures used, superior toughness was found by using the double austenitization treatment:

In another study performed on 420 stainless steel [14], grain refinement was achieved by the use of a high temperature prequench, along with a reduction in carbide size. They attributed the grain refinement to the second hardening occurring from the martensitic prior microstructure, similar to the prequenching theory described above:

To move on to a more typical knife steel with this type of high temperature prequench, I will cover a study on D2 [15]. The researchers performed a temper at 540°C prior to the second austenitize for unspecified reasons. All were given a final temper at 200°C. They found that using the double austenitization yielded an intermediate grain size similar to the 431 study, but also giving lower carbide volume due to the high temperature prequench. They did not perform any toughness testing. Interestingly they found that a lower second austenitize resulted in the highest hardness.

There are some reasons to question this particular study, other than the fact that no toughness testing was performed. The hardness of the single austenitized 1000°C seems quite low, 612 Hv is equivalent to about 54-55 Rc. If properly heat treated that should have been much closer to 60 Rc. The article also mentions that with the double austenitize material they detected the presence of “fine carbides along boundaries” which could be deleterious to toughness.

One more important study to cover is one performed on M2 [16]. They compared single austenitized conditions with a double austenitization with 1220°C as the first austenitizing temperature. In each case the hold time was 5 minutes followed by air quenching. They did not measure grain size but if the M2 prequench chart shown above can be used here, the second austenitizing temperatures should be below the discontinuous grain growth range, especially for 1050 and 1100°C, despite the high first austenitizing temperature of 1220°C. They found a decrease in toughness when using a double austenitize:

• Preheating helps to avoid distortion and quench cracks but is not necessary in all situations
• Finer grain size provides better toughness
• Prequenching is effective in refining the grain size somewhat because martensite is a better structure from which to form a fine austenite grain structure
• Multiple cycling can further refine the grain through nucleation of more austenite grains on the former boundaries
• A high temperature prequench has been reported to improve toughness through the reduction of carbide volume, but a study with M2 found a reduction in toughness

[1] https://www.uddeholm.com/files/PB_Uddeholm_vanadis_4_extra_english.pdf

[2] Krauss, George. Steels: processing, structure, and performance. Asm International, 2015.

[3] Grange, R. A. “The rapid heat treatment of steel.” Metallurgical transactions 2.1 (1971): 65-78.

[4] Grobe, Arthur H., George A. Roberts, and D. S. Chambers. “Discontinuous Grain Growth in High Speed Steel.” Trans. ASM 46 (1954): 759.

[5] Kula, Eric, and Morris Cohen. “Grain growth in high speed steel.” TRANSACTIONS OF THE AMERICAN SOCIETY FOR METALS 46 (1954): 727-758.

[6] TUJI, Katsumi, and Kiyoshi ARAO. “Discontinuous Grain Growth Phenomena and Grain Refining Treatments in High Speed Tool Steel.” Tetsu-to-Hagane 63.1 (1977): 80-89.

[7] Roberts, George Adam, Richard Kennedy, and George Krauss. Tool steels. ASM international, 1998.

[8] Grange, R. A. “Strengthening steel by austenite grain refinement.” ASM Trans Quart 59.1 (1966): 26-48.

[9] Grange, Raymond A., and Edward R. Shackelford. “Method of producing ultrafine grained steel.” U.S. Patent No. 3,178,324. 13 Apr. 1965.

[10] Verhoeven, John D. Steel metallurgy for the non-metallurgist. ASM International, 2007.

[11] YH, Kim. “Thermal mechanisms of grain and packet refinement in a lath martensitic steel.” ISIJ international 38.11 (1998): 1277-1285.

[12]   Rao, BV Narasimha, and G. Thomas. “Design of Fe/4Cr/0.4 C martensitic steels eliminating quench cracking.” Materials Science and Engineering 20 (1975): 195-202.

[13]  Balan, K. P., A. Venugopal Reddy, and D. S. Sarma. “Effect of single and double austenitization treatments on the microstructure and mechanical properties of 16Cr-2Ni steel.” Journal of materials engineering and performance 8.3 (1999): 385-393.

[14]  Srivatsa, Kulkarni, et al. “Improvement of impact toughness by modified hot working and heat treatment in 13% Cr martensitic stainless steel.” Materials Science and Engineering: A 677 (2016): 240-251.

[15]  Salunkhe, Sa, et al. “Effect of single and double austenitization treatments on the microstructure and hardness of AISI D2 tool steel.” Materials Today: Proceedings 2.4-5 (2015): 1901-1906.

[16]  Ögel, Bilgehan, and Erdogan Tekin. “The effect of double austenitization on the microstructure and fracture toughness of AISI M2 high speed steel.” steel research international 69.6 (1998): 247-252.