On the Optimization of Microstructure and Mechanical Properties of CrWMn Tool Steel by Deep Cryogenic Treatment

Abstract

The authors explore here the influence of cryogenic treatment on the microstructure and mechanical properties of CrWMn tool steel. Compared to the normal quenching and tempering process, the tensile strength, toughness, and hardness is enhanced after quenching‐cryogenic‐tempering treatment. The result is attributed to the high number density of precipitated ultra‐fine small granular carbide (SGC), the transformation of retained austenite, and the refinement of the martensite. Two types of retained austenite (filmy and blocky) would transform at low temperature or during the tempering process. Results of tempering curves indirectly prove that a part of SGCs precipitates during the deep cryogenic treatment. The wear resistance is improved and the friction coefficient is decreased by adding cryogenic treatment. The excellent wear resistance after cryogenic treatment is ascribed to the enhancement of hardness and low friction coefficient. The extra precipitation of small granular carbides induced by cryogenic treatment contributes to the wear resistance.

Quantification of the Dislocation Density, Size, and Volume Fraction of Precipitates in Deep Cryogenically Treated Martensitic Steels

“…Deep cryogenic treatment (DCT) is a sub-zero treatment where materials are exposed to temperatures below −125 • C with liquid nitrogen being the preferred cooling media [1]. Deep cryogenic treatment on martensitic steels is commercially carried out to improve wear resistance [2][3][4][5][6][7], and it has also been reported to affect other material properties such as hardness [8][9][10][11][12][13][14][15][16][17][18][19][20] and tensile strength [21][22][23]. There are some proposed mechanisms for these improvements in the literature.…”

“…The transformation of retained austenite (RA) to martensite is one of the primary objectives of DCT, and it is one of the predominating proposed mechanisms for improvement in wear resistance, as the martensitic phase is much harder than the parent austenite phase [2,4,19]. This transformation has regularly been quantified and verified via X-ray diffraction (XRD) [3,4,19,22,[24][25][26][27][28][29][30]. However, it has also been observed that alloys that are already fully martensitic can exhibit markedly improved wear resistance after DCT, even though there is no change to the martensite volume fraction [31]; this is commonly referred to as the conditioning of the martensite [32].…”

“…When DCT is applied, the usual process is to quench (or cool) the alloy to room temperature and then carry out the DCT, which is followed by a tempering treatment. The most commonly proposed mechanism for improvement in wear resistance other than the removal of RA after DCT is an increase in the volume fraction and homogeneity of secondary alloy carbides that develop during tempering [4,19,21,24,30,[32][33][34] or during the DCT itself [22]. Some work on tool steels has claimed that carbon atoms segregate to dislocations during the DCT [20,23,30,33,[35][36][37], and that it is these segregated atoms that contribute to the increase in carbides during the tempering process [28].…”

“…Some work on tool steels has claimed that carbon atoms segregate to dislocations during the DCT [20,23,30,33,[35][36][37], and that it is these segregated atoms that contribute to the increase in carbides during the tempering process [28]. However, other arguments state that the carbon atoms are essentially immobile at the temperatures used in DCT [38,39] and that it is in fact gliding dislocations that capture immobile carbon atoms [4,40], and some work has reported an elimination of fine carbide precipitation as a result of DCT [41]. Therefore, it remains unclear what effect DCT has on the subsequent tempering behavior and carbide development, with a range of results being published [2,39,42].…”

Increasing the Wear Resistance of Structural Alloy Steel 38CrNi3MoV Subjected to Isothermal Hardening and Deep Cryogenic Treatment

“…This is one of the theoretically explained mechanisms for wear resistance increasing because the martensitic phase has much higher hardness than the initial austenite phase [31]. Typically, this phase transformation has been investigated and quantified via the X-ray diffraction (XRD) method [32][33][34][35]. The authors of this work used such a method, along with an ultrasonic method and electron microscopy with a microanalyzer, to observe the transformations of RA.…”