Using the physically based constitutive model and processing maps to understand the hot deformation behavior of 2304 lean duplex stainless steel

RESEARCH DATA : The research data for this article are available on the University of Pretoria Bahr Dilatometer machine and the SEM post-processing machine, available on request.Please read abstract in the article.Mintek.https://link.springer.com/journal/11661hj2023Materials Science and Metallurgical Engineerin

Tracking the railway line in-service behaviour through metallurgical and mechanical property characterisation

Wear, contact-impact, fatigue and corrosion properties are important factors initiating rail track degradation and are composition dependent. This work focuses on the effect of chromium content on the metallurgical and mechanical properties of the the rail steels. Two medium steel railway tracks differing in their chromium content, alloy MS02 with 0.02 wt% Cr and alloy MS03 containing 0.72 wt% Cr, were characterised in order to predict their in-service performance as a function of their chemical composition. The microstructural analysis revealed in MS02 a relatively coarser grain size, consisting of pearlite and grain boundary ferrite. MS03 showed a fully pearlitic structure with no grain boundary ferrite. The average distance between cementite lamella in rail MS03 is shorter than that in MS02. The Rockwell C hardness values of MS02 were lower than those of MS03 which is in agreement with the microstructural analysis. As expected, a finer pearlitic structure observed in rail MS03 resulted in a superior tensile and impact properties as compared to MS02. These data indicate that MS03 with a fully pearlitic structure gives a better wear resistance and higher strength than MS02 and, it is anticipated to give a better corrosion and oxidation resistance due to higher chromium content.http://www.satnt.ac.zaam2022Materials Science and Metallurgical Engineerin

Study of one-step and two-step quench and partition heat treatments on a medium carbon high silicon alloy using dilatometry

This study evaluated the microstructural evolution in a medium carbon high silicon steel during one-step, and two-step quench and partition (Q&P) processes using dilatometry experiments. The two-step Q&P process was carried out using different quench temperatures ranging from 180 to 260 oC. In the one-step process, Q&P heat treatment samples were held isothermally for ten minutes after quenching at specified temperatures ranging between 200 and 450oC. The two-step Q&P process yielded a higher fraction of retained austenite than a one-step Q&P process. During the isothermal hold step, the volume expansion due to carbon partitioning and austenite decomposition behavior was interpreted by experimentally determined strain values. For the one-step Q&P process, the austenite decomposition kinetics above and below the Ms temperature differed, as evidenced by the JMAK parameters. The TTT diagram generated for the one-step Q & P process showed a “swing back” at a temperature of around 355oC.Ferrous Metal Development Network (FMDN).http://www.satnt.ac.za/index.php/satntam2022Materials Science and Metallurgical Engineerin

Effect of hot rolling parameters on recovery mechanism in 436 (17%Cr, Nb-Mo) ferritic stainless steel

Ferritic stainless steel (FSS) grades are widely used for applications that require high strength and corrosion resistance. Their cost and versatility in the steel market have attracted a lot of interest from both industry and academic research. Despite their desirable properties, these steels grades experience surface defects as a result of microstructural evolution that evolves due to softening mechanism. The current study simulated the roughing hot rolling processes of AISI 436 (17%Cr, Nb-Mo) FSS to study the effects of inter-pass time and strain on the precipitation behaviour and the softening mechanisms in 436 FSS. The softening mechanisms and the resulting microstructures were investigated using SEM-EBSD technique. The results revealed Particle Stimulated Nucleation of new grains during the simulated roughing rolling which promoted recrystallisation due to strain accumulation. Stored deformation energy was found to increase with an increase in interpass time and strain.University of Pretoria; MINTEK and Columbus Stainless Steel (Middleburg).http://www.satnt.ac.za/index.php/satntMaterials Science and Metallurgical Engineerin

Study of wear performance of wheel and rail steels under dry sliding conditions

The demand for efficient railway services has significantly increased in the past years due to an increased demand for the high-speed transportation of goods with high loads. The increase in loads and velocities has resulted in increased problems associated with rolling contact fatigue (RCF), rolling and sliding wear on the wheel and rail materials causing a reduction of service life of wheel/rail systems. Rail operating companies spend significant funds in maintenance and replacing damaged rails and wheels caused by wear. In addition, unscheduled maintenance due to wear and RCF often lead to poor availability of railway networks. For this study, dry sliding wear was investigated on wheel and rail steels using RTEC Multi-Function Tribometer. The results demonstrated that the rig was successful in simulating sliding wear, and that the fractions of the wear components could be varied, and it also provided instrumentation. Information on coefficient of friction against sliding distance and applied force were obtained which were used to compare sliding wear performance of both wheel and rail steels. The wheel was found to perform better than the rail under the same conditions due to its high initial hardness values and smaller interlamellar spacing.Mintekhttp://www.satnt.ac.za/index.php/satntam2022Materials Science and Metallurgical Engineerin

A study of smart materials for roofbolts application in the mining industry

M.Tech.This dissertation details the research performed into the development of a smart material for roofbolt application in the mining industry. It describes the methodology and research done to design a smartbolt using a metastable austenitic stainless steel. A number of measuring devices were used to study the properties and structure of this smartbolt alloy. These devices include a Ferritescope and a Krautkramer USM 25 DAC ultrasonic sound velocity measuring instruments. The dissertation details the development, processing, laboratory and field testing of the smartbolt alloy. The designed alloy was found to have much stronger workhardening effect, causing it to have a relatively low ductility. Therefore, the threads on the roofbolts produced from this alloy were machined instead of thread rolled. It was also found that the incubation strain of the smartbolt alloy to be a' transformation in the smartbolt alloy loaded in uniaxial and biaxial tension. It was found that the von Mises effective strain criterion gives a reasonable correlation of transformation kinetics. Using the modelling method, it was found that the failure strains for the rock are not the same as those for the smartbolt alloy. All the smartbolts installed in the haulage tunnel (level 94 of mineshaft) showed a pattern of increasing longitudinal ultrasonic velocity (load) with time. This was due to the fact that mining was taking place above the area (in level 93 of the mineshaft)

The Role of Retained Austenite on the Performance of High Chromium White Cast Iron and Carbidic Austempered Nodular Iron for Grinding Ball Applications

The role of retained austenite on the performance of cast iron based grinding media (balls) is somewhat controversial. One school of thought is that retained austenite improves the performance through work hardening and transformation to martensite. Others argue again that the same phenomenon compromises the performance of the balls through spalling. In this study, high chrome white cast iron (HCWCI) and carbidic austempered nodular iron (CANI) grinding balls were subjected to different heat treatments to yield various amounts of retained austenite, after which they were subjected to abrasive wear testing. The tests involved were high stress abrasion (pin-on-belt abrasion test), low stress abrasion (rubber wheel abrasion test according to ASTM G65), combined abrasion-impact conditions (ball mill) and impact tests (drop test). As expected, it was found that the percentage retained austenite increases with an increase in austenitising temperature due to the dissolution of carbides. The surface hardness again decreases as the destabilisation temperature and amount of retained austenite increases. The retained austenite content of the alloy before the high stress abrasion wear (pin on belt test, POB) was found to be significantly higher if compared to that after testing. The decrease in retained austenite was due to the transformation from austenite (rest) to strain induced martensite (’) that occurs during the high stress abrasion wear test. The austenitising temperature of 1000 C resulted in low percentage mass loss values or a lower wear rate from the ball mill test. The best high stress abrasive wear resistance for HCWCI, which was due to optimum properties, was achieved when the retained austenite content was reduced to 22.6%. It was found that the high stress abrasion resistance decreased abruptly once the retained austenite content decreased to lower than 10%. The CANI alloy which was austempered at 275 C (with graphite spheroids and carbides in the ausferrite matrix and 17.4% retained austenite) performed better under both low and wet abrasive wear (ball mill) conditions.Dissertation (MSc)--University of Pretoria, 2018.Materials Science and Metallurgical EngineeringMScUnrestricte

Efficiency of different commercial TiBAl grain refiners on refinement of pure aluminum cast structures

Industries largely rely on empirical tests to determine what composition and amount of TiBAl grain refiners to add during the casting of aluminum alloy for cast grain structure refinement. TiBAl grain refiners are aluminum master alloys having intermetallic particles within the aluminum matrix, these particles are mainly made of Ti and Al borides, having higher melting point than aluminum matrix. Addition of them to the melt, induces heterogeneous nucleation of aluminum cast grain structures, which promotes solidification of fine equiaxed grains at expense of columnar grain structures. This yields in grain refinement of cast structures, and is beneficial to mechanical and surface treatment properties on the final product. Very often in industrial practice during casting, whether a grain refiner is suitable for a given alloy, the amount to be added in the melt, and what cooling rates to use, are questions remain and reaching the balance becomes a cumbersome practice, and leads to pernicious material’s properties. This study aims to explore the refinement efficiency of three variants of locally supplied TiBAl grain refiners on the microstructure of cast pure aluminum (CPAl). A standard test method of TP-1 was adopted and the efficiency of TiBAl grain refiners was determined according to the average grain sizes. It was found that Al-3Ti-1B alloys from different suppliers yield different grain sizes, however, Al-5Ti-1B alloy has better efficiency than Al-3Ti-1B alloy system on CPAl material. The optimum holding time for CPAl melt at 750℃ is 5 minutes to achieve a fine grain size