Metalurgické pochopení vlivu kryogenního zpracování nástrojových ocelí
In the ´40ies of the last century, Cohen c.s. in the USA and Gulyaev c.s. in the USSR, independently conceived the idea to improve hardenable steel’s properties by subjecting it to a cryogenic treatment. Their activities departed from two observations reported as early as 1925 by Mathews:
• retained austenite is always present in medium- and high-carbon steels after hardening to room temperature;
• immersion of steel in cryogenic liquids reduces the fraction of retained austenite and increases hardness.
In the late ´70ies, R.F. Barron c.s. gave the proof that in particular their wear resistance, can be improved importantly by cryogenic treatment. They investigated the effect of various treatment parameters on wear resistance, among which, the cooling rate to cryogenic temperature, the lowest temperature reached during the treatment, the holding time at cryogenic temperatures, and showed that the holding time plays an important role on the effect of the treatment. They suggested that the (partial) transformation of retained austenite during treatment and the formation of fine carbides on subsequent tempering enabled by an unknown mechanism that takes place at cryogenic temperature are the mechanisms responsible for the improved wear resistance.
The debate on the mechanisms happening in steel at cryogenic temperatures and on how these influence the response to wear have continued for half century. Nevertheless, the metallurgical understanding of the microstructural changes involved in cryogenic treatment of steel is still premature.
The effect of cryogenic treatment on the properties of D2 and D3 tool steel well characterizes the debate. Among the steel investigated by Barron, D2 showed the most positive response to cryogenic treatment. In the early ´90ies, Collins and Dormer systematically investigated the effect of various treatment parameters on hardness and concluded that two mechanisms occur at cryogenic temperatures:
• athermal, i.e. independent of time, transformation of austenite into martensite;
• low temperature “conditioning of martensite”, an unidentified time-dependent mechanism that would be rate controlled by the diffusion of carbon and thereby promotes fine precipitation of carbides during reheating to, or above, room temperature.
About 25 years later, the latter mechanism was supported by Das c.s., who added quantitative information on the effect of the holding time at 77 K on the carbide population and on the resistance to wear. In the late ´90ies Mohan Lal c.s. considered D3 steel. The material was austenitized to minimize the fraction of austenite retained after cooling to room temperature and it was then cryogenically treated at 77 K for various holding times. The work showed that cryogenic treatment can improve the wear performance and that the holding time is of fundamental importance. They concluded that the, presumed athermal, martensite formation during cryogenic treatment cannot be the reason for improved wear resistance. During the last decade, the opposite conclusion was arrived at by Gavriljuk c.s. D2 steel was austenitized at particularly high temperature, significantly higher than specifications from the supplier, and time-dependent formation of martensite at cryogenic temperature was demonstrated. Moreover, it was revealed that the isothermal martensite formation at cryogenic temperature leads to the development of carbon clusters and modifies the crystallography of the carbides that precipitate on tempering. It was concluded that the time-dependent formation of martensite is the key factor to understand the effect of cryogenic treatment on tool steels.
The activity in our department aimed at clarifying the fragmentary and controversial picture that emerges from the literature.
Firstly, our activities focused on 100Cr6. Synchrotron XRD in combination with magnetometry showed that martensite forms during cryogenic treatment on cooling, on isothermal holding and on re-heating, and that the fraction transformed is maximal when cryogenic treatment is performed directly after quenching. It was also revealed that martensite formation evokes compressive stresses in austenite at temperatures higher than 135 K, whereas no compression builds up in austenite when martensite forms at T < 135 K. The compression on austenite was interpreted as a consequence of the elasto-plastic mechanical interaction between the austenite and martensite phases during transformation at temperatures above 135 K. Additionally, it was observed that cryogenic treatment facilitates the thermal decomposition of retained austenite, which starts at a temperature approx. 20 K lower in cryogenically treated specimens as compared to conventionally treated ones.
In the last 10 years, time-dependent martensite formation was investigated in numerous ferrous alloys. These included stainless steels, Fe-C and Fe-N. The investigation showed that time-dependent martensite formation in Fe alloys is the rule rather than the exception and certainly not an anomaly. In systems forming martensite with lath morphology, the formation of martensite is purely time dependent and can be suppressed on fast cooling to the boiling point of nitrogen, i.e. 77 K, followed by fast reheating to room temperature. In systems forming plate martensite, the twinned martensite cannot be suppressed but the dislocated martensite can be suppressed and is time dependent. Although time-dependent martensite formation is sluggish at 77 K, it is pronounced in the temperature range 100 – 230 K, which implies that cooling and heating rates to and from cryogenic temperature are important. The investigations also showed that isothermal holding at cryogenic temperature is effective to minimize the content of retained austenite in steel, provided that the treatment is extended for several, say dozens, of hours.
Investigation of D2 steel is our most recent activity. Treatment included various austenitization and tempering temperatures, to cover a broad spectrum of material´s conditions. Retained austenite was present in all samples after quenching to room temperature. It is shown that cryogenic treatment reduces the fraction of retained austenite, but does not eliminate it. Isothermal martensite formation takes place in the temperature range 100K – 220 K and is fastest at approx. 150 K, consistent with literature data from Gavriljuk et al. A cryogenic cycle including prolonged storage at boiling nitrogen temperature minimizes the fraction of retained austenite in the material, even though isothermal transformation at 77 K proceeds negligibly slowly. The isothermal formation of martensite is associated with magnetic softening, which can be interpreted in terms of rejection of carbon from solid solution in martensite, which supports Gavriljuk et al.´s work. Additionally, in situ investigation of tempering reaction shows that, prior to decomposition, which occurs beyond 770 K, retained austenite is enriched in carbon from martensite and further stabilized. This phenomenon happens in the temperature interval 475-625 K, which is the most applied range for tempering of D2 grade. These observations suggest that the volume fraction of retained austenite in the material has an influence on the volume fraction of carbides that precipitates in martensite. Prolonged isothermal holding at 77 K minimizes the fraction of retained austenite (carbon sinks), and thus maximizes the amount of carbon dissolved in martensite. This promotes the driving force for carbide precipitation in martensite during tempering (prior to austenite decomposition). Consequently, a larger volume fraction of carbides has to precipitate, a higher nucleation rate is achieved, i.e. finer carbides develop and carbide precipitation can occur at a lower temperature than in conventionally hardened steel.