Effects of Cold Work on Abnormal Grain Growth During Simulated Carburizing of 4121 Steel Containing Nb

Precision cold-forging processes are used to produce near-netshape parts that may then be carburized. During carburization thermal cycles, abnormal grain growth (AGG) after cold forging is known to develop microstructures which limit fatigue strength. In the present study, a small 0.04 wt.% Nb addition was made to a low-alloyed AISI 4121 steel containing 0.3 wt.% Mo. Subcritically annealed specimens were cold rolled (to simulate cold forging) at selected reduction ratios up to 50%, heated according to a simulated gas carburizing cycle at 930 °C, and water quenched to produce a final martensitic microstructure. The number density of abnormally grown grains increased rapidly as the cold rolling reduction ratio increased from 0 to 10%. With a further increase in reduction ratio, the extent of AGG decreased and was absent in samples subjected to the maximum reduction ratio of 50%. The evolution of fine (Nb, Mo)(C,N) precipitates at various stages of processing was characterized by thermodynamic calculations and electron microscopy and compared to the occurrence of abnormal austenite grain growth. The significance of these results for controlling AGG and thus optimizing fatigue performance in commercially-produced cold-forged and carburized components is discussed.

Austenite Grain Growth Analysis in a Welded Joint of High-Strength Martensitic Abrasion-Resistant Steel Hardox 450

The paper presents the results of tests of a welded joint of Hardox 450 steel, belonging to the group of weldable high-strength boron steels with increased resistance to abrasive wear. As a result of the conducted research, apart from the basic structural indicators, an attempt was made to determine the correlation between the grain size of the prior austenite in the characteristic weld zones and its basic mechanical properties, such as yield point, tensile strength, percentage elongation after fracture, reduction of area, and impact strength. The scope of research quoted above was carried out for a welded joint of the considered steel at delivery state (directly after welding), in the normalising annealed state, as well as in water-quenched state, using different austenitisation temperatures in the range of 900–1200 °C. The results obtained showed a large influence of the parameters of the applied thermal heat treatment on the selected structural and mechanical properties of the welded joint.

Effect of Nb on the Austenite Microstructural Homogeneity of Hot Rolled Rebars

While the role of Nb in flat rolling of low carbon steels has been investigated in many works, the information about the use of Nb in rebar rolling of higher carbon grades is more limited. Rebar rolling presents differences relative to flat rolling that can affect the role of Nb, such as the application of higher number of rolling passes, higher strain rates, lower interpass times, and, consequently, enhanced adiabatic heating. Increasing the number of passes can contribute to austenite grain refinement. However, the high finishing temperatures in rebar rolling can lead also to significant austenite grain growth and microstructural heterogeneity development before phase transformation. This phenomenon will directly influence the final grain size and can also lead to the appearance of second hard phases in the final product. One of the options to avoid austenite grain growth is to add microalloying elements that retard grain growth kinetics, either in solid solution or as precipitates. This can open new roles for the application of Nb in rebar rolling. To analyze this, in this work laboratory torsion tests were performed with two 0.2%C steels microalloyed with two different Nb contents (0.029% and 0.015%). Soaking temperatures from 1100°C to 1250°C were applied to obtain different amounts of Nb in solid solution before grain growth study. The study shows that not only finish rolling temperature and cooling time, but also reheating temperature and the amount of Nb remaining in the form of undissolved precipitates are important factors controlling austenite grain growth.

Influence of the Mg content on the austenite grain growth in heat-affected zone of offshore engineering steels considering TiN particle pinning and Mo solute drag effects

The submicrometre and nanometre particle characteristics, soluble element contents, and austenite grain growth behaviors in heat-affected zone of offshore engineering steels with 0.0002 (2Mg) and 0.0042 (42Mg) wt.% Mg during the simulated welding process were studied. With increasing the Mg content in steel from 0.0002 to 0.0042 wt.%, the submicron particles are decreased in number and size with their compositions evolved from TiN to TiN + MgO capped with Mo carbides, and the number density of small-sized nanoparticles increases and large-sized nanoparticles decreases. When the temperature is below 1250 °C, the grain growth rate of two steels is not much different due to the larger Mo solute drag effect in 2Mg and larger pinning force in 42Mg. When the temperature is 1250–1300 °C, the small-sized nanoparticles in 42Mg is more than that in 2Mg, resulting in the larger pinning force and smaller grain growth rate in 42Mg. When heated to 1300–1350 °C and soaked at 1350 °C for 300 s, since large quantities of particles smaller than the critical size (dcr) are dissolved, the grain growth rate in 2Mg is smaller than that in 42Mg due to the greater amount of the effective pinning particles and larger pinning force in 2Mg.