Severe tempering of bainite generated at low transformation temperatures

Abstract

The response of a strong bainitic-steel to tempering at high temperatures is investigated, because a softened state is sometimes necessary during the manufacture of engineering components, prior to the final heat-treatment which hardens the material. Thermodynamic calculations conducted to determine the maximum temperature at which the steel could be annealed without forming austenite were found to significantly overestimate the actual temperature at which austenite, ferrite and cementite (γ + α + θ) can coexist in equilibrium. As a consequence, the tempering temperature must be limited to below about 700 • C, which unfortunately necessitates many days of tempering in order to reduce the hardness to <250 HV. An alloy modification which may shorten this tempering time is suggested. One unexpected outcome is that the unintended heat treatment in the threephase field revealed useful information about the nucleation of pearlite on cementite particles. Colonies of pearlite started inevitably on particles of cementite located at the γ/α interfaces, rather than at similar particles enclosed completely by the austenite.

Tempering of Low-Temperature Bainite

“…Softening may also be necessary to allow thermo-mechanical processing. [10,11] Previous work has shown that these steels are relatively resilient to tempering. [1,8,12] The fine plate size provides the dominant strength contribution, so high hardness is maintained until coarsening of the bainitic ferrite occurs.…”

Hydrogen Susceptibility of Nanostructured Bainitic Steels

“…The tempering treatment applied is sufficient in this alloy to remove the retained austenite but not sufficient to subsequently coarsen the ferrite, and previous work shows that coarsening occurs after the austenite has decomposed and is associated with a large drop in hardness. [21,25,26,32] These calculations support a correlation between the amount of hydrogen at saturation and the predicted grain boundary surface area per unit volume, rather than simply to the volume fraction of austenite (see Table III). As a result of the displacive transformation to carbide-free bainite, the grain boundaries will be predominantly of a single orientation relationship.…”

“…After saturation with hydrogen, 7 ppmw was desorbed by heating to 673 K, increasing to more than 9 ppmw after introducing micro-cracks by heat treatment; this suggests that incoherent phases can be used to introduce strong trapping sites. [32,33] In the evolution curves observed, the presence of austenite is associated with a shift in the desorption peak to around 498 K (225°C) from 383 K (110°C). At saturation, the diffusible hydrogen level was similar in the three conditions observed, with evolution curve similar below 383 K (110°C).…”

Hydrogen diffusion and the percolation of austenite in nanostructured bainitic steel

“…The as-transformed structure of Alloy 2, shown in figure 3a, is coarser, with an average plate thickness of 83 nm. Bainitic steels, such as the alloys studied here, are resistant to tempering, with little reduction in hardness or austenite volume fraction below 450 • C and significant softening only taking place at temperatures above 500 • C [27][28][29]. This is reflected in the fact that the hardness remains high in both alloys, even after tempering at 500 • C for 1 h. However, the reduction in hardness following the tempering treatments signals the onset of austenite decomposition, as confirmed by X-ray analysis (table 3).…”

“…Previous work on nanostructured bainite has shown that the decomposition occurs during isothermal heat treatment at temperatures of 400 • C or above [27,29,38,39]. However, these experiments involve continuous heating, so dilatometric experiments were done to measure the temperature range over which the substantive decomposition of austenite occurs.…”