The effect of single and double quenching and tempering heat treatments on the microstructure and mechanical properties of AISI 4140 steel

This investigation is concerned to evaluate the effect of double quenching and tempering (DQT) with conventional quenching and tempering (CQT) heat treatment processes on microstructure and mechanical behavior of a commercially developed hot rolled AISI 4140 type steel. Comparison of microstructure and mechanical properties of DQT and CQT heat treated specimens have been established in details. Optical and scanning electron microscopies have been used to follow impurity concentration and microstructural changes, and their relation to the associated mechanical properties. The results indicate that the improvement of mechanical properties particularly impact toughness of DQT heat treated specimens is much higher than that of the CQT condition, and this observation is rationalized in terms of finer austenite grain size developed in the DQT condition providing much finer martensitic packets within the grains and a lower level of impurity concentration of sulfur (S) and phosphorus (P) near the prior austenite grain boundaries as well.     

The tempering behavior of granular structure in a Mn-series low carbon steel

The granular structure in a Mn-series low carbon steel composed of ferrite matrix and martensite-austenite islands does not exhibit temper brittleness which is quite different from common microstructures in steels. This characteristic facilitates the performance optimization through adjusting tempering temperature. A good combination of tensile strength (750-1000 MPa) and impact toughness (Aku, 138-154 J) can be obtained after quenching and tempering at 400 °C for a round billet with 250 mm in diameter.     

Investigation on AISI 304 austenitic stainless steel to AISI 4140 low alloy steel dissimilar joints by gas tungsten arc,electron beam and friction welding

This paper presents the investigations carried out to study the microstructure and mechanical properties of AISI 304 stainless steel and AISI 4140 low alloy steel joints by Gas Tungsten Arc Welding (GTAW), Electron Beam Welding (EBW) and Friction Welding (FRW). For each of the weldments, detailed analysis was conducted on the phase composition, microstructure characteristics and mechanical properties. The results of the analysis shows that the joint made by EBW has the highest tensile strength (681 MPa) than the joint made by GTAW (635 Mpa) and FRW (494 Mpa). From the fractographs, it could be observed that the ductility of the EBW and GTA weldment were higher with an elongation of 32% and 25% respectively when compared with friction weldment (19%). Moreover, the impact strength of weldment made by GTAW is higher compared to EBW and FRW.     

Failure of alloy steel socket-head cap screws used in offshore oil production

Low alloy quenched and tempered steels are widely used as screws and studs. In this work screws of AISI 4140 steel were used to bolt the Christma's tree to his cap in offshore oil and gas production. In the moment of the cap re-motion to perform a routine maintenance five 1″ screws failed. The screws worked under cathodic protection, which means that they must have undergone hydrogen pick-up. The investigation showed that the screws had been correctly heat treated by quenching and high temperature tempering, but the sulfur content and the level of inclusions were too high. As a consequence, the screws were highly susceptible to hydrogen induced cracking (HIC). On the other hand, the analysis of the fracture surface revealed that the main cracking mechanism was fatigue. The main recommendation resulted from this work is that the specification for new screws for this kind of service must be more restrictive to sulfur and inclusions contents.     

Low temperature fracture properties of DIN 22NiMoCr37 steel in fine-grained bainite and coarse-grained tempered embrittled martensite microstructures

An as-received (AR) DIN 22NiMoCr37 nuclear reactor pressure vessel steel has been heat treated for 1 h at austenitising temperatures of 1373 and 1473 K to obtain different austenite grain sizes. After austenitising, the samples were water quenched, tempered for 2 h at 923 K, water quenched and then held isothermally at 793 K for 180 h before final air-cooling. The AR condition had a tempered bainite microstructure and a prior austenite grain size of 30 μm, whereas the heat treated conditions were tempered martensite and had a prior austenite grain size of approximately 100 μm for the 1373 K condition and ‘extraordinary’ large austenite grains (>1 mm diameter) for the 1473 K condition. Their low temperature fracture properties were determined and were related to the susceptibility to segregation induced embrittlement. Despite the heat treated conditions having a larger prior austenite grain size compared to the AR condition, at a given testing temperature, the tempered martensitic 1373 K condition generally exhibited higher strength and higher fracture toughness values at 123 K. The heat treated conditions generally exhibited higher local fracture stress (σ_f) values in 0.2 mm blunt notch SE(B)-0.4T specimens at 123 and 77 K.     

Surface carburised AISI 8620 steel with dual phase core microstructure

Abstract

In this study, the production of dual phase steel structure in the core of surface carburised AISI 8620 cementation steel and the effect of martensite volume fraction on tensile properties have been investigated. For these purposes, surface carburised (~0·8 wt-%C) specimens were oil quenched from 900°C to obtain a fully martensitic starting microstructure. Then specimens were oil quenched from intercritical annealing temperatures of 731 or 746°C to produce dual phase steel structure in the core of specimens with martensite fractions of ~25 or ~50 vol.-% and nearly wholly martensitic microstructure at the surface. Generally, specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strengths but superior ductility without sacrificing surface hardness than those specimens with fully martensitic microstructure in the core produced by using conventional heat treatment involving quenching from 850 to 950°C. Also tensile strength increased and ductility decreased with increasing martensite volume fraction.     

Residual stresses and structural changes generated at different steps of the manufacturing of gears: Effect of banded structures

Banded ferrite-pearlite structures, and in general chemically inhomogeneous structures, react non uniformly to elevated temperatures during forging and/or subsequent heat treatment processes, affecting the final stress state (plastic deformation is required to accommodate dissimilar thermal expansion behavior for each phase) and consequently leading to distortions. These unpredicted distortions are one of the major causes of rejected components and components that need to be reworked, leading to production losses.The aim of the present research work is to study the effect of forging and different thermal treatments (normalizing, quenching and tempering), i.e., the effect of different steps of the manufacturing of gears, on the final residual stress state, microstructure and hardness of AISI 4140 steel, a material that frequently presents ferrite-pearlite banded structures coming from segregation of alloying elements (such as chromium and carbon). With this purpose, portions of a forged AISI 4140 steel ring have been subjected to different thermal treatments. Residual stresses, hardness and microstructure after each treatment (forging, normalizing, quenching and tempering) have been studied experimentally and compared with the predictions of FEM simulations of heat treatment processes.     

Evolution of microstructure and mechanical properties during quenching and tempering of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel

Effects of quenching and tempering treatments on the development of microstructure and mechanical properties of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel were investigated. Samples were austenitized at 1123–1323 K for 2400 s and oil quenched (OQ) to produce mixed microstructures. Tempering was carried out at 473–773 K for 2–3 h. Phase transformation temperatures were measured using dilatometer. The microstructures were characterized using optical and scanning electron microscope. SEM–EDS analysis was carried out to determine the type and size of non-metallic inclusions. Volume percent of retained austenite was measured by X-ray diffraction technique. Hardness, tensile properties, and impact energies were also determined for all heat treated conditions. Fractography of impact specimens were done using stereomicroscope and SEM. The results showed that newly developed steel exhibited peak hardness, yield strength, and tensile strength of about 600 HV, 1760 MPa, and 1900 MPa, respectively, when OQ from 1203 K and tempered in between 473 and 573 K, combined with adequate ductility and impact toughness. Decrease in hardness and strength was observed with increasing tempering temperature whereas the impact energy was stable up to 623 K, however, impact energy was found to decrease above 632 K due to temper martensite embrittlement.     

Effect of processing parameters on the evolution of dislocation density and sub-grain size of a 12%Cr heat resistant steel during creep at 650 °C

The influence of hot-deformation and tempering temperature on the microstructure evolution of a 12%Cr heat resistant steel during short-term creep at 80-250 MPa and 650 °C was investigated. Quantitative determination of dislocation density and sub-grain size in the initial microstructure and after creep was investigated by STEM-HAADF. A correlation between microstructure evolution and creep response is established. All crept samples showed a significant increase of sub-grain size and a reduction of dislocation density. Hot-deformed samples showed better creep strength than non hot-deformed samples due to homogenization of the microstructure. The tempering temperature affected the dislocation density and the sub-grain size evolution, influencing the creep behavior.     

低碳微合金直接淬火钢的组织与力学性能

为了提高低碳直接淬火钢的强韧性能,对一种低碳Nb-V微合金钢进行了轧后直接淬火(DQ)和再加热淬火(RQ)热处理实验,分析了低碳直接淬火钢的的强韧化机理.采用光学显微镜、透射电子显微镜、硬度计、拉伸试验机以及冲击试验机研究了轧后热处理工艺对低碳Nb-V微合金钢组织和力学性能的影响.结果表明,DQ工艺钢马氏体板条间距细小,含有较多的位错亚结构,因此具有较高的强度和韧性.DQ工艺钢马氏体中的大量位错,促进了碳化物弥散析出,产生了显著的二次硬化效果.由于基体中固溶的Nb、V等元素推迟淬火马氏体在回火过程中的各种转变,以及回火时析出的细小弥散碳化物抑制马氏体铁素体回复、再结晶过程,DQ工艺钢表现出较高的回火稳定性.     

Improvement of mechanical properties in a dual-phase ferrite–martensite AISI4140 steel under tough-strong ferrite formation

This paper has been concerned to investigate in details the mechanical properties of AISI4140 heat treatable steel under ferrite–martensite dual-phase (DP) microstructures in conjunction with that of conventional quench-tempered (CQT) full martensitic condition. For this purpose, a wide variety of ferrite–martensite DP samples containing different volume fractions of ferrite and martensite microphases have been developed using step quenching heat treatment processes at 600 °C for 20–55 s holding times with the subsequent hot oil quenching after being austenitized at 860 °C for 60 min in the same situation as to the CQT condition. The finalized tempering heat treatment has been carried out at 600 °C for 30 min for both of direct quenched full martensitic and DP samples in order to optimize the strength–ductility combination. Light and electron microscopes have been used in conjunction with mechanical tests to assess the structure–property relationships in the DP and CQT heat treated samples. The experimental results indicate that the DP microstructures consisting about 7% volume fraction of fine grain boundary ferrite in the vicinity of martensite are associated with excellent mechanical properties in comparison to that of CQT condition. These observations are rationalized in terms of higher carbon concentration of the remaining metastable austenite leading to the harder martensite formation on the subsequent hot oil quenching, and so developing much harder ferrite grains as a consequence of more constraints induced in the ferrite grains during martensitic phase transformation in the remaining austenite adjacent to the ferrite area. The higher martensite volume fraction in the vicinity of thin continuous grain boundary ferrite network has been associated with the harder ferrite formation, causing higher work hardening behavior in the short time treated DP samples. Moreover, it has been found that in order to optimize the mechanical properties of ferrite–martensite DP samples, two independently parameters should be simultaneously controlled: one is the ferrite volume fraction and the other is ferrite morphology.     

Effects of carburization on expected fatigue life of alloys steel shafts

The motivation of this work is to improve fatigue life of the alloys steel power transmission shafts of the palm oil screw press machines in a Malaysia palm oil company. In order to improve daily productions of the palm oil, a high extraction efficiency of the machine with bigger crushing forces are required. Consequently, the failures due to torsional fatigue would become susceptible to the shafts. The shaft materials of SAAB 709 which has nearest equivalent properties to AISI4140 were then taken for sequential treatments and examinations. Carburizing processes with different combinations of soaking temperatures of 850 °C and 900 °C and soaking times of 2 h and 4 h were carried out. Standard mechanical tests were then conducted for determining the hardness and the ultimate tensile strength of the specimens. Different soaking temperature and time would result in different carburizing depths that in turn influence the fatigue endurance limit under bending torsional loadings.     

Optimization of process parameters of boro-carburized low carbon steel for tensile strength by Taquchi method with grey relational analysis

The results of study on the boro-carburizing and boronizing of AISI 1015 steel on tensile strength was carried out by Taquchi-grey relational method. The orthogonal array L₉(3⁴) was used to conduct the experiment. The thickness of boride layer increased with increase in process temperature and time. The thickness of boride layers for boronized AISI 1015 steel was more than the pre-carburized and boronized AISI 1015 steel. The microhardness decreased with increase in distance from the surface to the core. However, the hardness gradient reduced gradually from the surface to the core in case of boro-carburized treatments compared to boronized treatments. The optimal process parameters and their levels for pre-carburized AISI 1015 steel are carbon content 0.45% at 950 °C temperature and 4 h process duration. The results revealed that process time, case carbon content and process temperature influenced the yield strength and % elongation. The ultimate strength is influenced by the process temperature, process time and carbon content. The process temperature was the most influential control factor that affects the tensile strength properties.     

Effect of tube spinning and subsequent heat treatments on strength,microstructure and residual stress state of AISI/SAE type 4140 steel

Abstract

In the present study, the effects of deformation percentage (23, 30, 50 and 66%) and subsequent stress relief and tempering heat treatments on the mechanical properties, residual stress state and microstructure of AISI/SAE type 4140 steel tubes manufactured by forward spinning were evaluated. Mechanical properties were determined by means of hardness and tensile tests. The tangential component of the surface residual stresses was determined by a slitting method. Plastic deformation of the metal during spinning refined and elongated the grains in the direction of metal flow, following a spiral path, resulting in improved mechanical properties. Tensile and yield strengths, as well as hardness, were all increased as a function of increasing percentage deformation. With stress relieving, the strength values were enhanced, whereas a slight decrease in hardness took place. Stress relieving did not change the microstructure considerably, whereas tempering resulted in a partially recrystallised microstructure, removing the effect of plastic deformation. The tangential residual stresses were tensile, and those of the as deformed tubes increased with an increasing amount of deformation up to 50%, then tended to decrease. The magnitude of the residual stresses decreased with stress relieving heat treatment, while tempering reduced the residual stresses to negligible levels.     

Microstructure and mechanical properties of 780 MPa high strength steels produced by direct-quenching and tempering process

Microstructure and mechanical properties of 780 MPa grade steel plate manufactured by conventional reheat-quenching and tempering (RQ-T) and direct-quenching and tempering (DQ-T) processes were investigated. The DQ process was found to enhance the hardenability of steel effectively so that tensile strengths of a range from 780 to 860 MPa have been achieved using DQ-T process, while tensile strength of about 770 MPa has been obtained from the RQ-T sample. In contrast, low temperature toughness of DQ-T samples was generally inferior to that of RQ-T sample, unless hot rolling and cooling processes were optimized in a controlled manner. For example, fracture appearance transition temperature (FATT) of DQ-T samples was varied in a range from –50°C to –120°C, while RQ-T specimens exhibited nearly constant FATT of about –80°C. The finish-rolling temperature (FRT) was one of potential process parameters to determine strength/toughness balance of the steel manufactured by DQ process, while the effect of FRT was closely associated with the cooling rate applied in the process. It has been demonstrated that, for the specimens quenched with a cooling rate higher than 20°C/sec, it may seem to be appropriate to adjust the FRT as low as possible in the non-recrystallization region. In contrast, for the specimens quenched with a low cooling rate of less than 10°C/sec, it may seem to be proper to apply higher FRT to obtain excellent strength/toughness balance of the steel.     

Improved low temperature mechanical properties of 0·4C–Cr–Mo–Ni steel through modified heat treatments

Abstract

The low temperature mechanical properties of 0·4C–Cr–Mo–Ni steel can be improved significantly by thefollowing treatments. Modified oil quenching (MOQ): interrupt quenching at 573 K just below the martensitic transformation temperature followed by short time tempering at 673 K (up-quenching) before oil quenching and subsequently 473 K tempering (after conventional 1133 K austenitisation). Modified austempering (MA): the same up-quenching treatment followed by austempering at 673 K and subsequently water cooling. Each modified treatment was compared with its corresponding conventional treatment. The MOQ treatment significantly improved the notched tensile strength of the steel with slightly increased 0·2%PS and UTS, owing to an increase infracture ductility over the temperature range 123–203 K and also improved the Charpy impact energy of the steel over the temperature range 203–373 K. As a result of the MA treatment, the 0·2%PS and UTS and the notched tensile strength were developed remarkably with little change of fracture ductility over the temperature range 123–293 K. This treatment also improved the Charpy impact energy of the steel over the temperature range 203–293 K. The beneficial effect of the modified heat treatments on the mechanical properties is briefly discussed in terms of a modified law of mixtures, fibre loading theory, and fracture profiles.

MST/1157     

Effect of direct quenching on the microstructure and mechanical properties of the lean-chemistry HSLA-100 steel plates

The influence of direct quenching on structure-property behavior of lean chemistry HSLA-100 steels was studied. Two laboratory heats, one containing Cu and Nb (C:0.052, Mn:0.99, Cu:1.08, Nb:0.043, Cr:0.57, Ni:1.76, Mo:0.55 pct) and the other containing Cu, Nb and B (C:0.04, Mn:1.02, Cu:1.06, Nb:0.036, Cr:0.87, Ni:1.32, Mo:0.41, B:0.002 percent) were hot-rolled into 25 and 12.5 mm thick plates by varying finish-rolling temperatures. The plates were heat-treated by conventional reheat quenching and tempering (RQT), as well as by direct quenching and tempering (DQT) techniques. In general, direct-quench and tempered plates of Nb-Cu heat exhibited good strength (yield strength ∼ 900 MPa) and low-temperature impact toughness (average: 74 J at −85 °C); the Charpy V-notch impact energies were marginally lower than conventional HSLA-100 steel. In Nb-Cu-B heat, impact toughness at low-temperature was inferior owing to boron segregation at grain boundaries. Transmission electron microscopy (TEM) and scanning auger microprobe (SAM) analysis confirmed existence of borocarbides at grain boundaries in this steel. In general, for both the steels, the mechanical properties of the direct-quench and tempered plates were found to be superior to reheat quench and tempered plates. A detailed transmission electron microscopy study revealed presence of fine Cu and Nb (C, N) precipitates in these steels. It was also observed that smaller martensite inter-lath spacing, finer grains and precipitates in direct-quench and tempered plates compared to the reheat quench and tempered plates resulted in their superior strength and good impact toughness.     

Effect of cryogenic treatment on the tensile behaviour of En 52 and 21-4N valve steels at room and elevated temperatures

This experimental study investigates the effects of cryogenic treatment on the tensile behaviour of En 52 and 21-4N valve steels at room and elevated temperatures. The materials are subjected to shallow cryogenic treatment (SCT) at 193 K and deep cryogenic treatment (DCT) at 85 K and the tensile behaviour is compared with that of the conventional heat treatment (CHT). The high temperature tensile test is conducted at 673 K (400 °C) and 923 K (650 °C) for the En 52 and 21-4N valve steels respectively. The ultimate tensile strength of the En 52 and 21-4N DCT samples show an enhancement of 7.87% and 6.76% respectively, over the CHT samples tested at the elevated temperature. The average yield strength of the En 52 DCT samples has an improvement 11% than that of the CHT samples when tested at room and elevated temperatures. The deep cryogenic treatment conducted at the optimized condition shows 7.84% improvement in the tensile strength for the En 52 valve steel and 11.87% improvement for the 21-4N valve steel when compared to the strength of the samples without the cryogenic treatment. A scanning electron microscopic analysis of the fracture surface indicates the presence of dimples and microvoid coalescence on the grain facets and interfaces of the cryo-treated specimens. The fracture surface of the deep cryo-treated 21-4N valve steel specimen shows a complete intergranular fracture with deep secondary cracks between the grains. On comparing the results of the percentage elongation, the cryo-treated samples show a smaller reduction in the elongation than that of the CHT samples. It is concluded that the precipitation of fine secondary carbide through cryogenic treatment is the reason for the improved strength and the reduction in elongation.     

Effect of layer thickness on thermal properties of multilayer thin films produced by PVD

Cr/CrN/CrAlN, CrN/CrAlN and Cr/CrN thin layers were deposited by PVD (Physical Vapor Deposition). The multilayers were obtained from the combined deposition of different layers Cr, CrN and CrAlN thick films on on AISI4140 steel and silicon substrates at 200 °C, and evaluated with respect to fundamental properties such as structure and thermal properties. Cr, CrN and CrAlN single layers were also prepared for comparison purposes. The structural and morphological properties of PVD layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with EDS + WDS microanalyses, stresses were determined by the Newton’s rings methods using the Stoney’s equation and surface hardening and hardness profiles were evaluated by micro hardness measurements. The XRD data and HRTEM showed that both the Cr/CrN, CrN/CrAlN and Cr/CrN/CrAlN multilayer coatings exhibited B1NaCl structure with a prominent reflection along (200) plane, and CrAlN sub-layer microstructures composed of nanocrystallites uniformly embedded in an amorphous matrix. The innovation of this work was to use the thickness of three different coating types to determine the thermal properties. Furthermore, an empirical equation was developed for the thermal properties variations with temperature of AISI4140 steel coated with different multilayer coatings. The thermal conductivity of CrAlN single layered was lower than the multilayer and the bulk material AISI4140. Moreover, the influences of structure and composition of the multilayer coatings on the thermal properties are discussed.The thermal conductivity of nanoscale thin film is remarkably lower than that of bulk materials because of its various size effects.     

Plasma nitriding of AISI 52100 ball bearing steel and effect of heat treatment on nitrided layer

In this paper an effort has been made to plasma nitride the ball bearing steel AISI 52100. The difficulty with this specific steel is that its tempering temperature (~170–200°C) is much lower than the standard processing temperature (~460–580°C) needed for the plasma nitriding treatment. To understand the mechanism, effect of heat treatment on the nitrided layer steel is investigated. Experiments are performed on three different types of ball bearing races i.e. annealed, quenched and quench-tempered samples. Different gas compositions and process temperatures are maintained while nitriding these samples. In the quenched and quench-tempered samples, the surface hardness has decreased after plasma nitriding process. Plasma nitriding of annealed sample with argon and nitrogen gas mixture gives higher hardness in comparison to the hydrogen–nitrogen gas mixture. It is reported that the later heat treatment of the plasma nitrided annealed sample has shown improvement in the hardness of this steel. X-ray diffraction analysis shows that the dominant phases in the plasma nitrided annealed sample are ε (Fe_2 − 3N) and γ (Fe₄N), whereas in the plasma nitrided annealed sample with later heat treatment only α-Fe peak occurs.