Showing papers by "John G. Speer published in 2013"

Quenching and Partitioning Steel Heat Treatment

Abstract: Quenching and partitioning (QP property advancements continue to be made through research on this emerging technology. Early investigations [1] also proposed a corresponding thermodynamic model for Q&P steel and its heat treatment, which is now referred to as constrained carbon equilibrium [3]. Since first proposed in 2003, Q&P steel has gained interest for its potential to enhance properties of strength and ductility with compositions similar to transformationinduced plasticity (TRIP) steel and has been proposed as a third-generation automotive steel (Fig. 1) [4]. Many researchers [5–17] have investigated the relationship between properties and microstructures of Q&P steels subjected to various heat treatments and showed that the ultrahigh strength of Q&P steel results from martensite laths, while its good ductility is attributed to TRIP-assisted behavior of retained austenite during deformation. De Moor et al. [14] examined the stability of retained austenite and showed that the TRIP effect occurs in Q&P steels, thereby effectively contributing to the significant strain hardening. Santofimia et al. [15, 16] and Takahama et al. [17] analyzed microstructural evolution during annealing Editor’s Note The following is a preview chapter from the upcoming volume Steel Heat Treating Fundamentals and Processes, Volume 4A, ASM Handbook, Jon Dossett and George Totten, editors. The volume is scheduled for publication later this year.

Effect of Testing Temperature on Retained Austenite Stability of Cold Rolled CMnSi Steels Treated by Quenching and Partitioning Process

Abstract: The effect of testing temperature on retained austenite (RA) stability of industrially cold rolled CMnSi sheet steel treated by quenching and partitioning (Q&P) process has been investigated by observing the deformation and transformation behavior of RA at different testing temperatures. Uniaxial tensile properties at different temperatures were determined and a correlation between RA stability and mechanical properties were also established. Ultimate tensile strength increases monotonously when temperature decreases, while total elongation reaches an optimum value between 0 and 20°C, where RA exhibits the greatest TRIP effect. Work hardening rate was calculated to decrease through three different stages in an oscillation manner, leading to significant enhancement in both strength and ductility. The kinetic of deformation-induced martensite transformation is also studied and the stability of RA can be evaluated by comparing the kinetic parameter β.

Anisotropy of Mechanical Properties of API-X70 Spiral Welded Pipe Steels

Abstract: The effects of microstructure and texture on the toughness anisotropy of two API-X70 pipeline steels were investigated. One steel contained no nickel (0Ni) and the other contained 0.3 wt pct nickel (0.3Ni). Charpy V-notch impact testing was conducted on plate samples for both steels in three directions: longitudinal (L), transverse (T), and diagonal (D) with respect to the rolling direction. The microstructures of both steels were mixed and consisted of acicular ferrite, granular bainite, and small amounts of polygonal ferrite, with martensite-austenite and retained austenite islands as secondary phases. The ductile to brittle transition temperatures (DBTT) for the Charpy impact test were higher in the D direction for both plates, with a pronounced increase in the 0Ni steel. The anisotropy in toughness was mainly attributed to the crystallographic texture.

Effects of Deformation Behavior and Processing Temperature on the Fatigue Performance of Deep-Rolled Medium Carbon Bar Steels

Abstract: The effects of processing temperature on the deep-rolling response of three medium carbon bar steels, a quenched and tempered 4140 alloy, a 0.34C, 1.21Mn, 0.66Si nontraditional bainitic alloy, and a 0.36C, 1.37Mn V-microalloyed ferrite plus pearlite steel, was assessed through bending fatigue. The significantly different deformation behaviors of the three alloys were characterized through standard and nonstandard quasi-static and cyclic uniaxial tension and compression tests at room temperature (RT) and in situ at temperatures up to 634 K. Deep rolling, performed at RT and at elevated temperature (HT) in the dynamic strain-aging (DSA) regime, increased measured endurance limits by 51-62 pct (RT) and 96-117 pct (HT) as compared with the baseline condition. The enhanced fatigue performance by RT deep rolling primarily reflected the effects of the introduction of favorable residual stresses. The improved fatigue performance from HT deep rolling was attributed to the enhanced resistance to strain reversal of the material deformed during deep rolling, due to a change in deformation mechanism from dislocation-interstitial interactions in the DSA regime during processing, which inhibited mechanically induced relaxation of residual stress during cyclic loading.

Microstructural Simulations via Thermal Processing of Roll Bonded Steel Laminates

Abstract: Laboratory processed laminated co mposites, designed to simu late co mpositional grad ients present in cast and hot rolled industrial microstructures, wereused to assess microstructural evolution during heat treating and the effects of specific microstructural variables on mechanical p ropertiesin steels. In this paper, two microstructural systems are presented to illustrate the broad applicability of the simulat ion technique. In one example, artificial compositional segregation of Mn was introduced to simulate microstructural banding observed in most industrial cast and hot rolled steels. Laminates were produced from two no minally 0.4 wtpct C steels with either 0.82 or 1.83 wtpctMn. The effects of co mposition band width and heat treatment, designed to alter the sharpness of the Mn gradients between layers, on microstructures andtensile p roperties are presented. In a second example, mu ltiphase microstructures characteristic of the next generation advanced high strength sheet steels (AHSS) were simulated by roll bonding alternating layers of a steel with 2 wtpctMn and another steel with 18 wtpctMn. The resulting mechanical properties correlate to predictions of a composite model when sufficient interfacial strength was achieved to prevent tunnel crack formation.