Showing papers on "Tempering published in 2013"

General relation between tensile strength and fatigue strength of metallic materials

Abstract: With the development of high-strength materials, the existing fatigue strength formulae cannot satisfactorily describe the relation between fatigue strength sigma(w) and tensile strength sigma(b) of metallic materials with a wide range of strength. For a simple but more precise prediction, the tensile and fatigue properties of SAE 4340 steel with the tensile strengths ranging from 1290 MPa to 2130 MPa obtained in virtue of different tempering temperatures were studied in this paper. Based on the experimental results of SAE 4340 steel and numerous other data available (conventional and newly developed materials), through introducing a sensitive factor of defects P, a new universal fatigue strength formula sigma(w)=sigma(b)(C-P.sigma(b)) was established for the first time. Combining the variation tendency of fatigue crack initiation sites and the competition of defects, the fatigue damage mechanisms associated with different tensile strengths and cracking sites are explained well. The decrease in the fatigue strength at high-strength level can be explained by fracture mechanics and attributed to the transition of fatigue cracking sites from surface to the inner inclusions, resulting in the maximum fatigue strength sigma(max)(w) at an appropriate tensile strength level. Therefore, the universal fatigue strength formula cannot only explain why many metallic materials with excessively high strength do not display high fatigue strength, but also provide a new clue for designing the materials or eliminating the processing defects of the materials. (c) 2012 Elsevier B.V. All rights reserved.

Microstructure, tensile and toughness properties after quenching and partitioning treatments of a medium-carbon steel

Abstract: The effect of different microstructures on the tensile and toughness properties of a low alloy medium carbon steel (0.28C–1.4Si–0.67Mn–1.49Cr–0.56Mo wt%) was investigated, comparing the properties obtained after the application of selected quenching and partitioning (Q&P) and quenching and tempering (Q&T) treatments. After Q&T the strength–toughness combination was the lowest, whereas the best combination was achieved by Q&P, as a result of the carbon depletion of the martensite and the high stabilization of the austenite. Nonetheless, the presence of islands of martensite/austenite (MA) constituents after Q&P treatments prevented the achievement of toughness levels comparable to the ones currently obtainable with other steels and heat treatments.

A carbide-free bainite/martensite/austenite triplex steel with enhanced mechanical properties treated by a novel quenching-partitioning-tempering process

Abstract: A novel quenching–partitioning–tempering (Q–P–T) process was employed in a Mn–Si–Cr alloyed steel to obtain a triplex microstructure comprising carbide-free bainite, martensite and retained austenite. The carbide-free bainite formed during the quenching step (rather than partitioning step) of Q–P–T process can enhance the mechanical properties (e.g. the product of strength and elongation, ∼31.4 GPa%) by extending the partitioning time up to 30 min. A good combination of strength and elongation (Ultimate tensile strength: ∼1655 MPa; Elongation: ∼19%) has been realized for the Mn–Si–Cr alloyed high strength steel without expensive alloying, e.g. Ni, Mo, Nb in previous Q&P/Q–P–T processes. The enhanced mechanical properties could be attributed to the various types of retained austenite in the Q–P–T treated carbide-free bainite/martensite/austenite triplex microstructure.

Steel heat treating fundamentals and processes

Abstract: Coverage on heat treating in the ASM Handbook series is being expanded into several volumes, and ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes is the first of multiple volumes on heat treating. Volume 4A introduces the basics of steel heat treating and provides in-depth coverage of the many steel heat treating processes. Coverage includes: Physical metallurgy of steel heat treatment Fundamentals of steel hardness and hardenability Practical aspects of hardenability as a key criterion in the selection of steel Hardenability calculations and the use of hardenability data Fundamentals and practical aspects of steel quenching Expanded coverage on quenching processes Updates and expansion on annealing, tempering, austempering and martempering New articles on cleaning, subcritical annealing, austenitising, and quench partitioning of steel heat treatment Significant expansion on the fundamental and applied aspects of surface hardening by applied energy, carburising, carbonitriding, nitriding, and diffusion coatings Editors and authors have also added charts, examples, and practical reference data for application purposes.

High strength low-carbon alloyed steel with good ductility by combining the retained austenite and nano-sized precipitates

Abstract: A Cu-bearing Nb-microalloyed bainitic steel with high yield strength (>700 MPa), good elongation (>35%) and low temperature toughness was developed for heavy plates. An intercritical annealing plus tempering (L–T) heat treatment was applied for combining reversed transformation with the precipitation during the tempering stage. The enrichment of alloying elements during the annealing process leads to the drop of A c1 temperature, which makes the reversed transformation and precipitation occur simultaneously in the subsequent tempering process. The secondary enrichment of Mn and Ni makes the reversed austenite stable enough to be retained at room temperature. About 29% retained austenite introduced by the L–T process contributes to excellent elongation more than 10% by the TRIP effect. The formation of Nb(C,N) and (Nb, Mo)C precipitates during the annealing and cooling stages inhibits the recovery of dislocations, while more Nb precipitates smaller than 10 nm and Cu precipitates with sizes of 20 nm play significant strengthening role on the matrix. The combination effects of retained austenite and nano-sized dual-precipitate increased yield strength, ductility and toughness remarkably.

Strengthening and Toughening of a Heavy Plate Steel for Shipbuilding with Yield Strength of Approximately 690 MPa

Abstract: HSLA-100 steel with high content of alloying elements (nominally in wt pct, 3.5 Ni, 1.6 Cu, and 0.6Mo) is now used to produce heavy plates for constructing a hull and drilling platform. We proposed here a substantially leaner steel composition (containing 1.7 Ni, 1.1 Cu, and 0.5Mo) to produce a heavy plate to 80 mm thickness with mechanical properties comparable with those of the HSLA-100 grade. A continuous cooling transformation (CCT) diagram of the steel was constructed. Key parameters of thermal treatment and revealing mechanisms of strengthening and toughening were derived based on industrial production trials. The microstructures of the 80-mm-thick plate were lath-like bainite (LB) at near surface of the quarter thickness (t/4), and granular bainite (GB)+LB at center thickness (t/2) after solutionizing and water quenching (Q). The effect of tempering (T) on the microstructures and properties of the plate was investigated. Excellent combination of room temperature strength and low-temperature Charpy V-notch (CVN) toughness approximately equivalent to that of the HSLA 100 grade (YS > 690 MPa, CVN energy >100 J even at 193 K [−80 °C]) was achieved in the plate treated by the QT process with tempering temperature of 898 K (625 °C). The combination of strength and toughness at t/4 is superior to that at t/2 of the plate under both as-quenched and QT conditions. This result is attributed to that the fraction of high-angle grain boundaries (HAGBs) at t/4 is higher than that at t/2.

Experimental verification of segregation of carbon and precipitation of carbides due to deep cryogenic treatment for tool steel by internal friction method

Abstract: This work presents an internal frictional behavior of cold work tool steel subjected to different heat treatment schedules to get insight related to segregation of carbon and refinement of carbide particles due to deep cryogenic treatment. The temperature dependence of internal friction was used to describe the variation of carbon concentration in solid solution of the martensite matrix in successive tempering steps. The results indicate that the carbon atoms segregated to nearby defects forming atomic clusters producing strong interactions, including interstitial carbon atoms themselves and between the interstitial carbon atoms with time-dependent strain field of dislocations because of lattice shrinking and thermodynamic instability of martensite during the deep cryogenic treatment. The clusters act as and grow into nuclei for the formation of fine carbide particle on subsequent tempering that was verified by analyses of TEM micrographs.

Enhanced Mechanical Properties of a Hot-Stamped Advanced High-Strength Steel via Tempering Treatment

Abstract: The hot stamping process has an extensive range of applications due to its advantages over the traditionally used stamping techniques developed in the past. To enhance the mechanical properties of the indirectly hot-stamped parts, the quenching and partitioning (Q&P) process has been recently applied on boron-alloyed steel. In the current research, it was observed that the tempering treatment on the directly hot-stamped boron steel resulted in better mechanical properties and higher formability index compared with the reported results using the Q&P process. The nano-carbide formation and the dislocation annihilation during the tempering treatment were suggested as the evident reasons for the occurrence of the mentioned robust properties. The ease of the practical implementation of the tempering route together with the markedly enhanced mechanical properties of the tempered parts make the suggested method privileged. Additionally, the variations in the yield strength before and after tempering were quantitatively evaluated.

Effect of cryogenic treatment on microstructure and wear characteristics of AISI M35 HSS

Abstract: Cryogenic treatment has been widely acknowledged as a means of improving wear resistance of tool materials. A Comparative study on conventionally heat treated and cryogenic treated AISI M35 grade high-speed steel specimens has been presented in this paper. Specimens initially subjected to conventional heat treatment at austenitizing temperature of 1200 C were subsequently subjected to shallow cryogenic treatment at -84 C for 8 hours and deep cryogenic treatment at –195 C for 24 hours followed by double tempering at 200˚C. Presence of retained austenite was studied at the end of each of the above treatment using XRD analyzer. An estimated 19% retained austenite present at the end of conventional heat treatment was reduced to 5% at the end of shallow cryogenic treatment, while deep cryogenic treatment practically removed all traces of austenite in the sample. Changes in the microstructure were studied using SEM. Fine precipitates of carbides of size 0.3-0.5µ were observed in cryogenically treated samples. Variation in mechanical properties such as toughness and hardness has been studied. There was no change in toughness due to cryogenic treatment and it corroborates well with the results of fractography. Wear characteristics were studied using pin on disc wear tester. The operative modes and mechanisms of wear have been identified as severe delaminative and mild oxidative from the morphology of worn surface of pin. The results unambiguously confirm enhancement in hardness and wear resistance of cryogenically treated specimens.

Bulk combinatorial design of ductile martensitic stainless steels through confined martensite-to-austenite reversion

Abstract: The effect of local martensite-to-austenite reversion on microstructure and mechanical properties was studied with the aim of designing ductile martensitic steels. Following a combinatorial screening with tensile and hardness testing on a matrix of six alloys (0–5 wt% Mn, 0–2 wt% Si, constant 13.5 wt% Cr and 0.45 wt% C) and seven martensite tempering conditions (300–500 °C, 0–30 min), investigations were focussed on martensite-to-austenite reversion during tempering as function of chemical composition and its correlation with the mechanical properties. While Mn additions promoted austenite formation (up to 35 vol%) leading to a martensitic–austenitic TRIP steel with optimum mechanical properties (1.5 GPa ultimate tensile strength and 18% elongation), Si led to brittle behaviour despite even larger austenite contents. Combined additions of Mn and Si broadened the temperature range of austenite reversion, but also significantly lowered hardness and yield strength at limited ductility. These drastically diverging mechanical properties of the probed steels are discussed in light of microstructure morphology, dispersion and transformation kinetics of the austenite, as a result of the composition effects on austenite retention and reversion.

The crystallography of M23C6carbides in a martensitic 9% Cr steel after tempering, aging and creep

Abstract: The orientation relationships of M23C6 carbides in a martensitic creep resistant steel were studied. Almost all M23C6 carbides were located at (sub)grain boundaries after tempering and aging. The carbides were slightly elongated along the boundary planes and obeyed the Kurdjumov-Sachs, Nishiyama-Wassermann, and Pitsch orientation relationships as well as two new orientation relationships, that is and , with α-Fe matrix. On the other hand, the M23C6 particles in the neck portion of crept specimen lost their orientation relationships with α-Fe.

Effect of thermo-mechanical treatment on microstructure and mechanical properties of P92 heat resistant steel

Abstract: The effect of thermo-mechanical treatment on the microstructure and mechanical properties of commercial P92 steel was investigated. The steels were warm-rolled in metastable austenitic state with reductions of 75 and 93%, respectively. Tempering in a certain temperature range from 650 to 765 °C for 1 h was subsequently used to optimize the size of precipitates and form a homogeneous steady-state dislocation structure through recovery process. The results showed that the microstructure was obviously refined and a great amount of nano-scale precipitates were increased significantly compared to the as-received material. The tensile strengths and microhardness were significantly improved by increasing plastic deformation and tempering at 650 and 700 °C, but were decreased dramatically after tempering at 765 °C. The creep tests were carried out at 600 °C under 210 MPa and at 650 °C under 160 MPa. The results demonstrated that the creep rupture life of the thermo-mechanical treated P92 steel was improved significantly. High density of homogeneously distributed precipitates reduced creep rates by pinning the grain boundaries and blocking the motion of dislocation.

Effect of welding speed on microstructure, hardness and tensile properties in laser welding of advanced high strength steel

Abstract: The increasing demand of lightweight and durability makes advanced high strength steel attractive for future automotive applications. In this study, 0·8 mm thick bare 600, 800 and 1000 MPa grade dual phase steel and 1500 MPa grade martensitic steel were laser welded, and the effect of welding speed on weld bead geometry, microhardness, microstructure and tensile properties was investigated. The steels exhibited similar weldability, and a critical welding speed for acceptable joint was determined as 25 mm s−1. A linear relationship of the hardness at fusion zone with carbon equivalent was observed, while carbon content showed a poorer linear fit. Heat affected zone (HAZ) softening increased with the steel grades due to the higher martensite volume fraction of the base metal in stronger steels. In addition, decrease of welding speed led to longer tempering time and consequently higher degree of HAZ softening. Correlations between tensile strength and hardness were also investigated.

Effect of Heat Treatment on Reversed Austenite in Cr15 Super Martensitic Stainless Steel

Abstract: The effect of different heat treatments on the reversed austenite in Cr15 super martensitic stainless steel was investigated. The experimental results indicate that the microstructure of the steel is composed of tempered martensite and diffused reversed austenite after quenching at 1050 °C and tempering from 550 to 750 °C. The volume fraction and size of reversed austenite increase with increasing tempering temperature and both of them reach the maximum value at 700 °C. The volume fraction and size of reversed austenite decrease when the temperature is above 700 °C. The transmission electron microscope (TEM) results indicate that the orientation relationship between tempered martensite and reversed austenite belongs to Kurdjmov-Sach (K-S) relationship.

Investigation of the mechanical stability of reversed austenite in 13%Cr–4%Ni martensitic stainless steel during the uniaxial tensile test

Abstract: The mechanical stability of reversed austenite at room temperature in two 13%Cr–4%Ni low carbon martensitic stainless steel samples after different heat treatments has been investigated. The uniaxial tensile tests indicate that the reversed austenite resulting from the one-stage and two-stage intercritical tempering heat treatment have different mechanical stability, which induces distinct strength-ductility balance of the material. Experiments and crystal plasticity finite method simulations reveal that the special grain orientation relationship between the reversed austenite and martensite matrix, besides chemical composition, plays important roles on the mechanical stability of the reversed austenite. It is found that in both samples the Nishiyama–Wassermann or Kurdjumov–Sachs relationship between the reversed austenite and the martensite matrix provides an easy way for the active slip systems in austenite to penetrate the phase boundary to the adjacent martensite. This results in a high mechanical stability of the reversed austenite. In addition, the larger austenite grains in the second-stage tempering sample have higher mechanical stability in the martensitic steel due to the favored austenite stabilizing elements distribution behavior, and the favored stress distribution originating from the intrinsic strengths of the austenite and martensite matrix.

Thermomechanical warm forging of Ti–V, Ti–Nb, and Ti–B microalloyed medium carbon steels

Abstract: Conventional quenched and tempered steels have long been used to fabricate mechanical components, such as plates, shafts, and axles. All of these components are mainly fabricated employing medium carbon hot forged steels in the quenched and tempered condition. In the last decade, thermomechanical forging at intermediate (warm) temperatures of microalloyed medium carbon steels is increasingly adopted to reduce or eliminate quenching and tempering heat treatments in hot forged components. However, the use of these steels is still limited, being difficult to achieve a toughness comparable to that of fully heat treated ones. In an effort to improve toughness, an experimental campaign was performed to assess strength and toughness properties of three selected microalloyed medium carbon steels after thermomechanical forging simulations at warm temperatures, performed on a thermal and mechanical Gleeble 3800 ® apparatus. Microstructures and mechanical properties of these steels are shown to be highly dependent upon steel composition and thermomechanical schedule. The experimental results suggest that thermomechanical schedules can be designed to produce microalloyed medium carbon steels with refined microstructures, and therefore to realize improved strength and machinability; yet, toughness is still the most critical property as compared to that of quenched and tempered steels.