Investigation into microstructure of spray formed V4 cold working mould steel and its roiling and annealing

The material selected for this work was the spray formed Vanadis4 high alloy cold working mould steel （abbreviated to V4 steel）. Its microstructure, hot rolling process, and annealing treatment have been investigated. Observed from the optical and electron microscopes, the as-sprayed V4 steel had the finer microstructure of uniform and equiaxial grains ,while after hot rolling for densification and spheroidized annealing, the V4 steel obtained an excellent spheroidized structure that is favorable to subsequent quenching and tempering treatment. The spheroidized structure and level of annealed hardness of the V4 steel are almost the same as expensive imported powder metallurgy the V4 steel. It is difficult to produce V4 steel with the conventional ingot metallurgical technique, so the multi-step and high-cost powder metallurgy method is generally used at present. Compared to the powder metallurgy technique, using the spray forming technique to produce the V4 steel has obvious advantages and potential market competitiveness in reducing production costs, simplifying working process, and shortening the production cycle.

Carbide Evolution in High Molybdenum Nb-microalloyed H13 Steel during Annealing Process

Based on the Thermo-Calc thermodynamic software, the type of equilibrium precipitated carbides and their contents in high Mo Nb-microalloyed H13 steel （NMH13 steel） were calculated. The composition, morphology, and distribution of carbides after spheroidal annealing of two forged experimental steels were comparatively examined by means of optical microscopy （OM）, scanning electron microscopy （SEM）, electron dispersive spectroscopy （EDS） and transmission electron microscopy （TEM）. VC, M23 C6 and M6C are identified in H13 steel after spheroidizing annealing, while （V,Nb）C, M23C6 , M2C and M6C are observed in NMH13 steel. Moreover, it is found that the ad- dition of Nb significantly enhances the stability of MC phase and the high Mo content accelerates the precipitation of small rod-shape M2C phase in NMH13 steel. The amount of the fine carbides in NMH13 steel obviously increased with M2 C and M6 C precipitated from the ferrite phase, which is in accordance with the results of thermodynamic cal- culations.

Effect of Subcritical Annealing Temperature on Microstructure and Mechanical Properties of SCM435 Steel

The effect of subcritical annealing temperature on microstructure and mechanical properties of SCM435 steel was investigated through changing the heating and soaking temperature as 660 °C, 680 °C, 700 °C, 720 °C and 745 °C. The microstructure and mechanical properties of intercritically annealed specimens were analyzed. With increasing the subcritical annealing temperature from 660 °C to 720 °C, the spheroidization ratio gradually increased, and the mechanical properties, formability and Vickers hardness were improved. According to the comprehensive comparison of mechanical properties and formability, the subcritical process at soaking temperature of 680-720 °C could achieve similar annealing effect as that of intercritical process. Therefore, the subcritical annealing temperature could be set as 700 °C in practice, with the Ac1 temperature fluctuation within ±20 °C, and the applicability and stability of subcritical annealing were guaranteed in industrial application. The plant results of the cold heading showed that the subcritical annealing could replace original intercritical annealing successfully with significantly saving time and energy.

Evolving Mechanism of Eutectic Carbide in As-cast AISI M2 High-speed Steel at Elevated Temperature

The evolution in type, size and shape of carbides in as-cast American Iron and Steel Institute （AISI） M2 high-speed steel before and after annealing were investigated. The micromechanism which was responsible for those changes was also analyzed and discussed. At the initial stage of reheating, metastable M2C-type carbide decomposed continuously. M6C-type carbide nucleated at the interface of M2C/γ firstly and grow from surface to center. Then MC-type carbide nucleated at both surface of M6C/M6C and inner of M6C. With the increasing decomposition of the metastable M2C-type carbide, the rod-shaped construction of eutectic carbide began neck- ing, fracturing and spheroidizing gradually. Held enough time or reheated at higher temperature, particle-shaped product aggregated and grew up apparently, while secondary carbide precipitated in cell and grew up less sig- nificantly than the former. Based on the above microstructural observation, the thermodynamic mechanism for decomposition of M2C carbide, for spheroidization of products, and for the growth of particles were analyzed. The rate equations of carbides evolution were derived, too. It shows that the evolving rate is controlled by diffusion coefficients of alloy atoms, morphology of eutectic carbides and heating temperature.