A novel approach to envisage effects of boron in P91 steels through Gleeble weld-HAZ simulation and impression-creep

This paper induced a novel methodology for the characterization of creep behavior of weld heat-affected zone (HAZ) for boron-free P91 (PM) and boron modified P91B (B-PM) steels. Gleeble-3800 thermo-mechanical simulator replicated specimens, representing coarse-grain HAZ (CGHAZ), fine-grained HAZ (FGHAZ), and inter-critical HAZ (ICHAZ). Short-term impression creep tests were conducted at 625°C/270-410MPa on PM/B-PM and their simulated HAZs after being subjected to post-weld heat treatment (PWHT) of 760°C/3 h. Microstructural characterization and local strain analyses were accomplished by electron back-scattered diffraction. Simulated microstructures of P91B-FG/ICHAZ after PWHT exhibited lath martensitic structure and large prior-austenite grain size as regards P91-FG/ICHAZ, correspondingly. Average values of local microstructural strain from local average misorientation were relatively high in B-PM and P91B-ICHAZ than PM and P91-ICHAZ, respectively. Similar observations were found for P91-CG/FGHAZ with their counterparts. Stress dependent steady-state creep-rate (SSCR) followed power-law for all specimens except PM. The minimum and maximum ranges of SSCR for P91B specimens were observed to be in a narrower range than P91 specimens. The value of stress exponent for all specimens was evaluated, and corresponding mechanisms were discussed. The analyses of microstructures and corresponding impression creep behavior of P91/P91B samples suggested that modification of 100 ppm boron to P91 steel improved creep-rupture ductility that delayed type IV failure at outer HAZ of P91 steel weldments.

Austenite Stability and Deformation Behavior in Medium Mn Steel Processed by Cyclic Quenching ART Heat Treatment

This study investigated the austenite stability and deformation behavior of cyclic quenching-austenite reverse transformation processed Fe-0.25C-3.98Mn-1.22Al-0.20Si-0.19Mo-0.03Nb medium Mn steel. A number of findings were obtained. Most importantly, the extent of the TRIP effect was mainly determined by an appropriately retained austenite stability rather than its content. Simultaneously, chemical elements were the key factors affecting austenite stability, of which Mn had the greatest impact, while the difference of retained austenite grain size and Mn content resulted in different degrees of retained austenite stability. Additionally, there were still large amounts of strip and granular-retained austenite shown in the microstructure of the CQ3-ART sample after tensile fracture, revealing that the excessively stable, retained austenite inhibited the generation of an extensive TRIP effect.

The qualitative–quantitative approach to microstructural characterization of nanostructured bainitic steels using electron microscopy methods

Both qualitative and quantitative analyses play a key role in the microstructural characterization of nanobainitic steels focused on their mechanical properties. This research demonstrates various methods of microstructure analysis using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) techniques, taking into account these two approaches. The structural constituents have been qualitatively characterized using TEM and selected area electron diffraction (SAED), together with quantitative analysis based on the misorientation angle (EBSD). Besides, quantitative measurement of austenite with both blocky and film-like morphologies has been carried out. Due to the scale of nanostructured bainite, it is also important to control the thickness of bainitic ferrite and film-like austenite; hence, a method for measuring their thickness is presented. Finally, the possibility of measuring the prior-austenite grain size by the EBSD method is also demonstrated and compared with the conventional grain boundary etching method. The presented methods of qualitative and quantitative analyses form a complementary procedure for the microstructural characterization of nanoscale bainitic steels.

Fatigue Performance of Low Pressure Carbonitrided 20MnCr5 and SAE 8620 Steel Alloys

Low pressure carbonitriding and pressurized gas quenching heat treatments were conducted on four steel alloys. Bending fatigue tests were performed, and the highest endurance limit was attained by 20MnCr5+B, followed by 20MnCr5, SAE 8620+Nb, and SAE 8620. The differences in fatigue endurance limit occurred despite similar case depths and surface hardness between alloys. Low magnitude tensile residual stresses were measured near the surface in all conditions. Additionally, nonmartensitic transformation products (NMTPs) were observed to various extents near the surface. However, there were no differences in retained austenite profiles, and retained austenite was mostly stable against deformation-induced transformation to martensite during fatigue testing, contrasting some studies on carburized steels. The results suggest that the observed difference in fatigue lives is due to differences in chemical composition and prior austenite grain size. Alloys containing B and Nb had refined prior austenite grain sizes compared to their counterparts in each alloy class.

Effect of Thermomechanical Rolling of the Induction Hardenability of a Micro-Alloyed 1045 Steel

A micro-alloyed 1045 steel was commercially rolled into 54 mm diameter bars by conventional hot rolling at 1000 °C and by lower temperature thermomechanical rolling at 800 °C. The lower rolling temperature refined the ferrite-pearlite microstructure and influenced the microstructural response to rapid heating at 200 °C·s-1, a rate that is commonly encountered during single shot induction heating for case hardening. Specimens of both materials were rapidly heated to increasing temperatures in a dilatometer to determine the Ac1 and Ac3 transformation temperatures. Microscopy was used to characterize the dissolution of ferrite and cementite. Continuous cooling transformation (CCT) diagrams were developed for rapid austenitizing temperatures 25 °C above the Ac3 determined by dilatometry. Dilatometry and microstructure evaluation along with hardness tests showed that thermomechanical rolling reduced the austenite grain size and lowered the heating temperature needed to dissolve the ferrite. With complete austenitization at 25 °C above the Ac3 there was little effect on the CCT behavior.

On a Modified Approach of Measuring Quench Severity and its Application

Instrumented methods for measuring the coefficient of heat transfer are difficult to implement in industrial quench systems. In 1985 Roy Kern presented a simple empirical method for calculating the quench severity of commercial quench systems using measured Jominy hardenability and a mid-radius (r/R=0.5) hardness of a 3-inch diameter 8640 or 4140 steel bar. A more general approach using the Kern methodology is presented here with hardness profile matching to determine the quench severity. Experiments were performed using 2-inch diameter bars of 8620 with a length to diameter ratio of 4. Test bars and Jominy bars were heat-treated following ASTM A255. Test bars were quenched using an experimental draft tube with a water velocity of 6 ft/s. An excel workbook was programmed to calculate the quenched hardness profile as a function of quench severity using prior austenite grain size and steel chemistry. Measured Jominy hardness and calculated hardenability were in good agreement provided the prior austenite grain size was incorporated into the calculations. Both the Kern method and hardness profile matching produced a quench severity equal to 1.45.

Effect of Initial Microstructure on the Toughness of Coarse-Grained Heat-Affected Zone in a Microalloyed Steel

Toughness of the coarse-grained-heat-affected-zone (CGHAZ) strongly depends on the prior austenite grain size. The prior austenite grain size is affected not only by chemical composition, thermal cycle, and dissolution of second-phase particles, but also by the initial microstructure. The effect of base metal microstructure (ferrite/pearlite obtained by air cooling and martensite obtained by water-quenching) on Charpy impact toughness of the CGHAZ has been investigated for different heat inputs for high-heat input welding of a microalloyed steel. A welding thermal cycle with a heat input of 100 kJ/cm and 400 kJ/cm were simulated on the MMS-300 system. Despite a similar microstructure in the CGHAZ of both the base metals, the average Charpy impact energy for the air-cooled base metal was found to be higher than the water-quenched base metal. Through thermo-kinetic simulations, it was found that a higher enrichment of Mn/C at the ferrite/austenite transformation interface of the CGHAZ of water-quenched base metal resulted in stabilizing austenite at a lower A1 temperature, which resulted in a coarser austenite grain size and eventually lowering the toughness of the CGHAZ.

Developments towards a Multiscale Meshless Rolling Simulation System

The purpose of the present paper is to predict the grain size of steel during the hot-rolling process. The basis represents a macroscopic simulation system that can cope with temperatures, stresses and strains of steel in a complete continuous rolling mill, including reversible pre-rolling and finishing rolling with several tenths of rolling passes. The grain size models, newly introduced in the present paper, are one-way coupled to the macro-scale calculations performed with the slice model assumption. Macroscale solution is based on a novel radial basis function collocation method. This numerical method is truly meshless by involving the space discretization in arbitrarily distributed nodes without meshing. A new efficient node generation algorithm is implemented in the present paper and demonstrated for irregular domains of the slice as they appear in different rolling passes. Multiple grain size prediction models are considered. Grain size prediction models are based on empirical relations. Austenite grain size at each rolling pass as well as the ferrite grain size at the end of rolling are predicted in this simulation. It is also shown that based on the rolling schedule, it is highly likely that recrystallization takes place at each pass throughout a continuous rolling mill. The simulation system is coded as a user-friendly computer application for industrial use based on programing language C# and an open source developer platform .NET and runs on regular personal computers The computational time for a typical rolling simulation is usually less than one hour and can thus be straightforwardly used to optimize the rolling mill design in a reasonable time.