Enhancement of the AISI 5140 Cold Heading Wire Steel Spheroidization by Adequate Control of the Initial As-Rolled Microstructure

The effect of the initial microstructure and soft annealing temperature on cementite spheroidization and microstructure softening is studied on an AISI 5140 hot-rolled wire. In coarse pearlite microstructure (λ: 0.27 μm), the cementite spheroidization progresses slowly under subcritical treatment, and the microstructure does not achieve the minimum G2/L2 IFI rating defined in the ASTM F2282 to be used in cold forming operations under any of the annealing treatment studies. Fine pearlite (λ: 0.10 μm) and upper bainite microstructures are more prone to spheroidization, and the minimum G2/L2 IFI rating is achieved under subcritical annealing at 720 °C for 6 h. Independent of the initial microstructure, even in the case of martensite, low hardness values within 165–195 HV are attained after imposing a 10 h long treatment at 720 °C. Annealing treatments conducted at 660 °C and 600 °C on pearlitic microstructures give rise to very poor softening. The G2/L2 rating is not achieved in any of the treatments applied at these two temperatures in this study. In pearlitic microstructures, the spheroidization progresses according to a fault migration mechanism, enhanced by the presence of defects such as lamella terminations, holes, and kinks. In the upper bainite, the row-like disposition of the cementite along the ferrite lath interface provides necks where dissolution and consequent lamellae break-up take place quickly under annealing.

Single-Step Heat Treatment for the Restoration of Mechanical Properties of Cold-Strained Mining Support Steel 31Mn4

The steel 31Mn4+QT630 is used frequently for mining support arches. The supports are cold strained, first during service, and again by re-rolling prior to reinstallation, which results in strain hardening and a loss of ductility. Consequently, many of the rerolled arch-segments are not suitable for reinstallation because their mechanical properties are inadequate. The objective of this work was to assess the feasibility of restoration of the required mechanical properties by means of a cost-efficient single step heat treatment. Specimens were cold deformed to different degrees in the range 0 % to 45 % to establish the relation between the degree of cold deformation and the hardness. Differently strain hardened specimens were subjected to subcritical annealing at temperatures 450 °C to 700 °C in the duration 0.5 h to 8 h to determine a suitable time-temperature combinations. Microstructures and mechanical properties were investigated of as-received, cold strained and recrystallized specimens. Tests performed were optical microscopy, scanning electron microscopy, tensile tests, hardness measurements and Charpy impact tests. Elongation at break of the as-received material was below the requirements of the applicable standard, and its microstructure contained significant fractions of pre-eutectoid ferrite and pearlite. Upon cold straining, hardness increased by approximately 2 HV per 1 % of strain. After one-hour recrystallization at 600 °C to 620 °C, the microstructure consisted of a ferritic matrix containing evenly dispersed globular carbide particles. The original ductility was restored, while the elongation, yield strength, and hardness were higher than in the as-received condition. These results confirmed that it is feasible to restore the original mechanical properties of heavily strained profiles with an adequate single-step heat treatment. Furthermore, they indicated that it should be possible to recover the required properties of inhomogeneously strained material with the same set of well optimized heat treatment parameters. Consequently, it should be possible to increase the reuse rate and to decrease the costs for new support arches significantly.

New Approach for Multiplying the Strength and the Formability of Cold Rolled BCC Based Structure Steel through Ultra Fine Structure Technique

Reduction in grain size of bcc based structure steel is still highly concerned in the cold rolled sheet to attain superior mechanical properties. As long as, the reduction of weight is much considered in the structure purposes, the strength/weight ratio of steel is highly demanded. In this study, an innovative technique was applied to attain ferrite grain size with hundreds of nanometer, in tandem with preserving the mechanical properties. In this approach, the micro-alloyed low carbon steel resulted from the thermomechanical process was followed by subcritical annealing regime prior to the first critical transformation temperature. To identify the effect of a micro-alloying element as vanadium, and the effect of subcritical annealing regime on the low carbon steel, two low carbon steel was subjected to studying in this research. The results refer that applying a subcritical annealing regime for the micro-alloyed low carbon steel after hot compression at intercritical annealing temperature can lead for attaining hundreds of nanometer ferrite grain size, which has a powerful effect on promoting the strength of the steel to exceed 1200 Mpa, in one hand with preserving the formability up to 20% as uniform elongation. Unexpectedly, the fine grain size obtained after the innovative technique promotes the impact toughness at room temperature, which is attributed to the fineness and the spheroid morphology of the secondary phase in conjugation with bcc ferrite structure.