Papers by Keyword: Secondary Hardening

Abstract: Temper bead welding is one of effective repair welding methods in case that post weld heat treatment is not easily applied. In order to evaluate the effectiveness of temper bead welding, hardness in HAZ becomes important factor. The neural network-based hardness prediction system of HAZ in temper bead welding for A533B low-alloy steel has been constructed by the authors in the previous study. However, for HAZ hardness prediction of other steels, it is necessary to obtain hardness database for each steel which is time-cost consuming, if the same method is used. The present study has been conducted to develop the generalized hardness prediction method applicable for other steels by utilizing the hardness data-base of A533B steel assuming that the hardness in HAZ of steels after tempering have a linear relationship with LMP (Larson-Miller parameter). On using the newly proposed extended method, only a few hardness data-base for the other steels is needed to obtain. Hardness distribution in HAZ of temper bead welding for other steels was calculated by using the extended hardness prediction system. The thermal cycles used for calculation were numerically obtained by a finite element method. The experimental results have shown that the predicted hardness is in good accordance with the measured one for steels without secondary hardening. It follows that the currently proposed extended method is effective for estimating the tempering effect during temper bead welding for the steels without secondary hardening.

Abstract: The main aim of this work was to study the behavior of the secondary hardening of AISI M3:2 high speed steel named Sinter 23^® produced by powder metallurgy process of hot isostatic pressing (HIP). The M3:2 high speed steel Sinter 23^® was submitted to heat treatment of hardening with austenitizing temperatures of 1140 oC, 1160 oC, 1180 oC and 1200 oC and tempering at 540 oC, 560 oC and finally 580 oC. The effectiveness and response of the heat treatment was determined using hardness tests (Vickers and Rockwell C hardness) and had its property of secondary hardness evaluated. The results showed that the secondary hardening peak of Sinter 23^® high speed steel (tempering temperature at which maximum hardness is attained) is at 540 °C for the lower austenitization temperatures of 1140 °C and 1160 °C, and it is at 560 °C for the higher austenitizing/quenching temperatures of 1180 °C and 1200°C.

Abstract: Development of new steels with higher creep resistance must be based on detailed knowledge of strengthening and degradation processes, structural stability at high temperature exposure and their welding. This paper is concerned with the real weld joint hardness and microstructure of progressive low alloy creep resistance T24 steel. These steel is widely used in many power-plant constructions all over the Europe. Welding the T24 steels components is one of the most important technological procedures. The aim of the performed investigation is to evaluate the hardness and microstructure in the heat affected zone (HAZ) and the weld metal. Results of the secondary hardening in the real weld joints after low temperature post weld heat treatment (460°C/48h) and without post weld heat treatment at service temperature are described in this paper. Obtained results are compared to the hardness values of the weld joint post weld heat treated at recommended temperature (740°C). The microstructure of the real weld joints was analysed by transmission electron microscopy (TEM) in order to identify minority phases - MX nanoparticles and/or coarse (M₂₃C₆) particles. Secondary hardening of the not heat treated and low temperature heat treated T24 weld joint was proved.

Abstract: The influence of Mo and V contents on the microstructure and secondary hardening of 6Cr6W2MoV steel was studied in this paper, and the process factors and the transformation mechanism of the influence results were discussed by phase equilibrium thermodynamic analysis. The results show that, the microstructure of 6Cr6W2MoV steel is uniform and thin, in which the undissolved carbide size after quenching is 0.23μm. In the heating and cooling process of the annealing, the dissolution and precipitation of M₂₃C₆↔M₆C carbides transformation can be used to carbides distributing uniform refinement. Adding an overmany Mo content will lead to coarsening of carbide particles due to small amplitude of M₂₃C₆↔M₆C transformation. Adding an overmany V content will appear a small of the bulk MC carbide on the crystallization, so the heterogeneity of carbides is increasing. The tempering hardness of 6Cr6W2MoV steel is 63.5HRC, and when adding an overmany Mo content the maximal hardness arrives to 65HRC. Adding Mo changes the carbide composing, it is conducive to the variety of carbides precipitation strengthening near intermediate temperature and the tempering hardness increasing. When adding an overmany V content the more MC carbide well occupy more amount of C, the tempering temperature dispersion increases too, so the tempering hardness debases deservedly and the highest hardness only reaches HRC60.5.

Abstract: 440C martensitic stainless steels are widely used because of their good mechanical properties. The mechanical properties of 440C martensitic stainless steel were evaluated after heat treatment of these materials at various types of heat treatment processes. The initial part of this investigation focused on the microstructures of these 440C steels. Microstructure evaluations from the as-received to the as-tempered condition were described. In the as-received condition, the formations of ferrite matrix and carbide particles were observed in this steel. In contrast, the precipitation of M₇C₃ carbides and martensitic structures were present in this steel due to the rapid quenching process from the high temperature condition. After precipitation heat treatment, the Cr-rich M₂₃C₆ carbides were identified within the structures. Moreover, a 30 minutes heat-treated sample shows the highest value of hardness compared to the others holding time. Finally, the tempering process had been carried out to complete the whole heat treatment process in addition to construct the secondary hardening phenomenon. It is believed that this phenomenon influenced the value of hardness of the 440C steel.

Abstract: High carbon steel stainless steel such as 440C martensitic stainless steel, are commonly used for automotive components, such as ball bearings, races, gage blocks and valve. In this study, 440C steel was coated with ZrO₂ by electrolytic deposition in ZrO(NO₃)₂ aqueous solution. After annealing, the ZrO₂ coated specimens were characterized by x-ray diffraction (XRD) and scanning electron microscope (SEM). Scanning electron micrograph showed that thickness of the coated sample was approximately 0.7µm. Besides that, secondary hardening effect occurred on the annealed SS440C substrate and it might be due to the presence of secondary carbide.

Abstract: Along with increasing W and Mo contents in Cr-W-Mo-V high alloy medium-upper carbon steels, the maximal hardness of secondary hardening during tempering is increasing gradually and arrives to 66.8HRC, and the congruent quenching temperature and the tempering temperature corresponding to maximal hardness are ascending. The quenching microstructure of experimental steels is matrix and a small quantity of undissolved carbides when the hardness is maximal, wich is corresponding to tempering temperature of remnant austenitic decomposing acutely. The precipitation of M₆C and MC carbides was detected, and M₇C₃ and M₃C carbides was detected too. But M₂₃C₆ carbide did not appear and M₂C carbide was detected undistinguishably. The temperature range of tempering maximal hardness is 500°C-550°C, and an exact temperature is opposite to the mass fraction ratio of equilibrium carbide phases at the temperature. The tempering hardness value can be obtained from H_S= a(1+b)/(0.0127a+0.00297), in which a is square root of saturation level of the carbon in the matrix and b is correction factor having something to do with alloy elements of carbide precipitation.

Abstract: A new kind of die steel, WD1 used for warm work dies, was developed. The secondary hardening behavior and its affecting factors were studied. The results show that the secondary hardness increases with the content of C, Mo and V elements and Si element can significantly improve the secondary hardening effect though it’s not one of secondary hardening elements. The hardness of WD1 steel tempered at 520°C is more than HRC60, which can meet the requirements of warm work dies.

Abstract: Applying the severe deformation, the mechanical properties related to secondary hardening were investigated. The Mo-Cr-Co-Ni steels containing (5-13)wt% Co and (8-14)wt% Ni were severely rolled at 850 °C, followed by direct quenching(DQ). DQ specimens were then isothermally aged at 475°C. Ni additions promoted aging kinetics, accompanied by a little enhancement in hardness. In contrast, Co additions enhanced the peak hardness, but did not induce a remarkable aging acceleration. In the 13Co addition group, an actual secondary hardening, that is, the peak hardness is higher than the as-quenched hardness. Variation in peak hardness(Rc) in alloys is summarized as follows; 14Ni-13Co(57.2) ≒ 11Ni-13Co(57.1) > 8Ni-13Co(56.5) > 11Ni- 9Co(56.2) > 8Ni-9Co(54.4) > 11Ni-5Co(53.4).

Abstract: Secondary hardening and fracture behavior in the high Co-Ni steels containing W, has been studied in terms of severe deformation, Cr addition, and austenite condition. Two kinds of Co- Ni steels, containing the only W(W steel), and the W and Cr(WCr steel), were severely rolled and followed by direct quenching(DQ). In comparison with undeformed specimens, reaustenitizing( RA) was performed. DQ specimens exhibited higher impact toughness as well as higher hardness, as compared to RA specimens. The aging kinetics of the WCr steel was accelerated due to the Cr addition. While the W steel showed the intergranular embrittlement in the high temperature RA condition, in addition, the WCr steel exhibited the transgranular mode of mostly dimple type.