Densification and microstructural evaluation during laser sintering of M2 high speed steel powder

Abstract

In the present work, the densification and microstructure of M2 high speed steel powder processed by direct laser sintering method was studied. Test specimens were produced using a 200 W continuous wave CO₂ laser beam at different scan rates ranging from 50 to 175 mm s^?1. The building process was performed under argon and nitrogen atmospheres in order to evaluate the role of sintering atmosphere. It was found that the sintered density strongly depends on the laser scan rate and thus on the duration time of the laser beam on the surface of the powder particles. Generally, with a decrease in the scan rate higher densification was obtained. However, formation of large cracks and delamination of the sintered layers is feasible at low scan rates. The results also demonstrated that sintering under argon atmosphere yields better densification compared to a nitrogen atmosphere, in particular at higher scan rates. The microstructure of laser sintered parts consisted of large and elongated pores parallel to the building direction. The metal matrix structure was found to be heterogeneous, i.e. carbon rich austenite was formed due to carbon segregation. This structure consisted of fine cellulars or dendrites of martensite and retained austenite. This article describes the influence of manufacturing parameters on the densification of laser sintered M2 high speed steel powder. The microstructural features of the processed parts are also addressed.     

Solidification microstructures and mechanical properties of vertical centrifugal cast high speed steel

Abstract

This study was undertaken to investigate the influence of Nb and V alloying elements and manufacturing conditions on the microstructural behaviour and mechanical characteristics of HSS (high speed steel) roll manufactured by a VCC (vertical centrifugal casting) process. In the Fe - 2C - 6Cr-1.5W - 3Mo - 4V alloy, the amount of MC carbide was increased and the the amount of M₇C₃ carbide decreased with an increase in V and Nb content. In steel containing 3%Nb, primary NbC carbide was formed within the cell in the matrix. The hardness of steel containing 6.5%V but no Nb was increased a little but when 9%V was added, the hardness decreased in the specimen owing to the soft ferritic matrix. The hardness of the matrix in steel containing 1.5%Nb increased, but decreased for 3%Nb addition. In wear tests, wear loss decreased with increasing rotational wear speed.     

Effect of electromagnetic centrifugal casting on solidification microstructure of cast high speed steel roll

Using self‐made electromagnetic centrifugal casting machine, optical microscopy (OM) and D/max2200pc X‐ray diffraction, the solidification microstructure and phases of as‐cast high speed steel(HSS) roll made by sand casting, centrifugal casting and electromagnetic centrifugal casting were investigated. The experiment results show that the phases of as‐cast high speed steel (HSS) roll are alloy carbide (such as W₂C, VC, Cr₇C₃), martensite and austenite. The centrifugal casting and electromagnetic centrifugal casting can apparently improve the solidification structure of HSS roll. With the increase of electromagnetic field intensity (B), the volume fraction of austenite in the HSS solidification structure increased obviously and eutectic ledeburite decreased, the secondary carbide precipitated from the austenite is more fine and distribution of secondary carbide is more even.     

A study of centrifugal cast high boron high speed steel

In the present study a high‐boron high speed steel (HSS) roll material was designed. Many expensive alloy elements have been substituted by cheap boron alloy, and high‐boron high speed steel roll has been manufactured by centrifugal casting method. The microstructures, mechanical properties and wear resistance of centrifugal casting high‐boron high speed steel roll have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), and X‐ray diffraction (XRD) analysis, hardness test, impact test and wear test. The results indicated that the solidification microstructures of high‐boron high speed steel roll consisted of M₂(B,C), (W,Mo)₂(B,C), M₃(B,C), M₂₃(B,C)₆ type borocarbides and martensite, a small amount of retained austenite. Borocarbides were continuously distributed over the grain boundary. After quenching from 1050 °C, local broken network appeared in partial borocarbides, and fine secondary borocarbide precipitated from the matrix. After tempering from 525 °C, the amount of precipitated borocarbide increased significantly. After heat treatment, the hardness of high‐boron high speed steel roll excelled 60 HRC, and its impact toughness excelled 8.0 J/cm². The single groove steel rolling amount of high‐boron high speed steel rolls increases by 500% than that of bainite cast iron roll, when the rolls are used in K₁ mill housing of bar mill.     

Investigation of quenching effect on mechanical property and abrasive wear behaviour of high boron cast steel

Abstract

The effect of quenching temperature from 900 to 1050°C on the microstructures, mechanical properties and abrasion resistance of modified high born cast steel containing 0·3 wt-%C and 3·0 wt-%B was studied by optical microscopy, scanning electron microscopy, X-ray diffraction analysis, tensile tester, impact tester, hardness tester and abrasion tester. Quenching at 900°C resulted in structures containing a small amount of pearlite. The existence of pearlite led to poor hardness and wear resistance of modified high born cast steel. Quenching at temperatures between 900 and 950°C resulted in the decrease in pearlite and the increase in hardness and abrasion resistance in comparison with 900°C quenching. The metallic matrix all transformed into the martensite quenching at 1000°C; the modified high born cast steel had high hardness, tensile strength, impact toughness and excellent abrasion resistance. The hardness, tensile strength and impact toughness of modified high born cast steel had no obvious change quenching over 1000°C. The increase in quenching temperature also led to the transformation of boride from continuous shape to isolated shape and promoted the coarsening of boride.     

Microstructure,hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel

Abstract

The present investigation reports on a study that has been taken up to develop an understanding of the electron beam welding characteristics of similar and dissimilar combination of maraging steel and high strength low alloy steel, which are in the hardened condition, i.e. maraging steel, in a solution that was in treated and aged condition, whereas high strength low alloy steel in a quenched and tempered condition before welding. The joint characterisation studies include microstructural examination, microhardness survey across the weldment and measurement of residual stresses. Maraging steel weld metal is under compressive stress rather than tensile stress as observed in low alloy steel welds because the martensite transformation occurs at a relatively low temperature. It has been observed that, in dissimilar metal welds, tensile stress is observed at the fusion boundary of low alloy steel and weld metal, whereas compressive stress is obtained at the location between weld and maraging steel fusion boundary. Dissimilar weldment contains a soft region beside the interface on maraging steel side because of the diffusion of manganese from low alloy steel towards maraging steel. The observed residual stresses, hardness distribution across the similar and dissimilar metal welds are correlated with the observed microstructures.     

Electron backscattering diffraction study of coalesced bainite in high strength steel weld metals

Abstract

Coalesced bainite is a coarse constituent recently found to develop along with the classical martensite, lower and upper bainite in steel weld metals. Its crystallography has been characterised using electron backscattering diffraction in combination with field emission gun scanning electron microscopy. It is confirmed that coalesced bainite grains are crystallographically homogeneous but do contain orientation gradients. The misorientations across different grains of coalesced bainite and relative to conventional bainite have also been studied. The observations are discussed in the context of the mechanism by which coalesced bainite evolves.     

Ultrahigh strength hot rolled microalloyed steel: microstructure and properties

Abstract

A low carbon steel alloyed with Ni, Mn, Mo, Cu and microalloyed with Nb and Ti was prepared. Continuous cooling transformation behaviour of the steel was evaluated. Formation of polygonal or Widmanstätten ferrite is suppressed at high temperature and the 'C' curve is shifted to an extreme right. At lower temperatures a flat top 'C' curve with a mixed structure of bainite and martensite was obtained and the transformation temperatures do not vary much with a wide range of cooling rates. The steel was thermomechanically processed at different finishing temperatures and ultrahigh strength values were obtained as a result of austenite grain refinement, highly dislocated fine lath martensite structure along with tiny precipitates of microalloying carbide and carbonitride at all finish rolling temperatures. The stable and large TiN/TiCN particles formed during casting have impaired the impact toughness values at ambient and at ?40°C temperatures.     

Effects of heat treatment processes on microstructure and properties of 2·25Cr–1Mo–0·25V HSLA steels

Abstract

In the present paper, the effects of the heat treatment processes with two conditioning treatments and four quenching–tempering processes on the mechanical properties of 2·25Cr–1Mo–0·25V high strength low alloyed (HSLA) steel are investigated. The results show that the conditioning treatments have obvious effects on the low temperature impact energy but little effect on the tensile strength. The elevation of the final austenitising temperature increases the strength, whereas it results in the decrease in the low temperature impact energy due to the coarse microstructure. The results of the fracture surfaces analysis further make sure that the fracture surfaces of tensile specimens all exhibit ductile characters with a lot of dimples. However, the fracture surfaces of impact specimens exhibit two typical fracture characters, i.e. the ductile and brittle fracture surface corresponding to the fine and coarse microstructures respectively. In addition, the elongation and reduction in area seem to be insensitive to the heat treatments. Meanwhile, the impact fracture mode is more sensitive to the grain size and not to the low temperature impact energy.     

Effect of tempering temperature on microstructure and properties of aluminium-bearing high boron high speed steel

Heat treatment is of great significance to the performance improvement of high speed steel. Via heat treatment, the microstructure of high speed steel can be improved, thus greatly improving the material performance. The effect of tempering temperature on the microstructure of aluminium-bearing high boron high speed steel (AB-HSS) was investigated by optical microscope (OM), scanning electron microscope (SEM) and x-ray diffraction (XRD). The hardness and wear resistance of the alloy at different tempering temperatures were tested by Rockwell hardness tester, micro-hardness tester and wear tester. The experimental results indicate that the tempering microstructure of aluminium-bearing high boron high speed steel consists of α-Fe, M₂B and a few of M₂₃(C, B)₆. Tempering temperature could greatly affect the wear resistance of materials. With the increase of tempering temperature, the wear resistance of aluminium-bearing high boron high speed steel firstly increase and then decrease. The alloy tempered at 450 °C has the best wear resistance and minimum wear weight loss. This study provides a reference for the formulation of heat treatment process of aluminium-bearing high boron high speed steel.     

高速钢-Q235钢轧制复合板的组织与性能

采用合适的工艺方法制备了两种高速钢(W18Cr4V,W6Mo5Cr4V2)与Q235钢的轧制复合板材,用剪切试验方法测定了复合板材的界面结合强度,用光学显微镜和扫描电子显微镜观察了复合板材界面结合状态.复合板材界面结合强度达到460MPa,界面结合良好,能够满足复合刀片对材料性能的要求.     

Reaction sintering and joining nickel aluminide to AISI 316 stainless steel by self-propagating high temperature synthesis

Abstract

Self-propagating high temperature synthesis (SHS) is a process whereby reactants are ignited to spontaneously transform to products in an exothermic reaction. The aim of this study is to propose a method to join nickel aluminide with AISI 316 stainless steel by SHS and to study the combustion synthesis of nickel aluminide. From the heat of combustion synthesis junctions were formed between annular AISI 316 stainless steel and a powder metallurgy compact of Ni and Al blends. The Al mole ratio for testing the joining grade in the initial powder mixture varied from 25 at.-% to 40 at.-%. In order to check the sufficiency of the SHS reaction, the test temperature was compared with the thermodynamic calculation values. The metallographic analysis indicated that NiAl and Ni₃Al were formed in the joint layer.     

Vacuum hot roll bonding of titanium alloy and stainless steel using nickel interlayer

Abstract

Vacuum hot roll bonding of titanium alloy and stainless steel using a nickel interlayer was investigated. No obvious reaction or diffusion layer occurs at the interface between stainless steel and nickel. The interface between titanium alloy and nickel consists of an occludent layer and diffusion layers, and there are the intermetallic compounds (TiNi₃, TiNi, Ti₂Ni and their mixtures) in the layers. The total thickness of intermetallic layers at the interface between titanium alloy and nickel increases with the bonding temperature, and the tensile strength of roll bonded joints decreases with the bonding temperature. The maximum tensile strength of 440·1 MPa was obtained at the bonding temperature of 760°C, the reduction of 20% and the rolling speed of 38 mm s^–1.     

Comparison of intensity of high temperature surface damage for 20MoCrS4 steel with varying parameters of dynamical heating

Abstract

When implementing new progressive forming technologies and thermomechanical treatments, it is necessary to pay attention to side effects, which negatively influence the semiproduct quality. During heating to the forming temperature in hot or warm forming operations, semiproducts are exposed to high temperatures in direct contact with corrosion environment for a certain period of time. Using protective atmospheres in mass production is unsuitable for economical reasons. To minimise the influence of heating on the surface degradation, it is necessary to reach the desired temperature and uniform temperature field as quickly as possible, yet in compliance with physical properties of the heated material and with the technology in use. The experiments were focused on investigation of various temperatures and heating times simulating conditions of dynamical and classical processes. The comparison of the influence of heating temperature and time on the iron scale occurrence intensity as well as the influence on the surface layers of the semiproducts was performed.     

Comparison of ASME K IR curve and dynamic master curve in determining ductile–brittle transition temperature of A48P2 steel

Abstract

The ductile–brittle transition temperature (DBTT) of grade A48P2 steel is characterised based on the American Society of Mechanical Engineers (ASME) fracture toughness K _IR curve and dynamic master curve approaches. The indexing parameter for the K _IR curve, reference temperature RT_NDT, is determined from drop weight and Charpy tests to be ?45°C. The dynamic master curve is constructed following ASTM standard E1921 guidelines; however, instead of precracked tests, the dynamic fracture toughness K _Jd is determined from Charpy V notch tests using a modified Schindler's procedure. A Weibull plot is constructed using the K _Jd data, and it is found that the points comply reasonably with the forced fit line of slope 4. The reference temperature for constructing the dynamic master curve, termed T^dy,Sch₀, thus determined is ?56°C. The ASME K _IR curve is shown to be conservative compared with the dynamic master curve constructed using T^dy,Sch₀.     

Tensile fracture behaviour of microstructurally engineered Cu bearing high strength low alloy steel

Abstract

An attempt has been made to highlight the influence of precipitation and microstructural constituents on tensile fracture behaviour in Cu bearing HSLA 100 steel. Variations in the microconstituents have been incorporated in the steel by engineering the microstructures through thermal treatments consisting of solutionising, water quenching and aging at various temperatures. The microstructure in quenched condition consists of mainly lath martensite, bainite and acicular ferrite besides little amount of retained austenite, carbides and carbonitrides. Aging up to 500°C facilitated fine coherent ?-Cu precipitation that lost its coherency at >550°C. Simultaneously, recovery and recrystallisation of martensite and acicular ferrite occurred at higher temperatures. The formation of new martensite islands occurred on aging at >650°C. Carbides, carbonitrides and retained austenite remained essentially unchanged. Tensile tests were conducted at a slow strain rate to study the tensile fracture behaviour of the steel. Microstructural and fractographic evidences indicating that coherent Cu precipitate causes the brittleness in the material in initial stages of aging whereas loss of coherency of Cu precipitate in later stages results in the reappearance of ductility in the material.     

Influence of post-weld heat treatments on microstructure and mechanical properties of gas tungsten arc maraging steel weldments

Abstract

The response to post-weld heat treatment of an 18%Ni (250 grade) gas tungsten arc weld metal has been investigated. The post-weld heat treatments are (a) direct aging at 480°C/3 h/air cooling, (b) solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling and (c) homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling. Metallographic characterisation of fusion zone revealed pronounced segregation of titanium and molybdenum along the interdendritic and intercellular boundaries. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Solutionised treatment at 815°C does not remove the segregation. Homogenisation treatment (1150°C/1 h/air cooling) succeeded in making the composition become homogenised. Mechanical properties including tensile, hardness and impact toughness were evaluated. Tensile test results showed that directly aged weldments exhibited lower strength but higher ductility than the other cases; this was attributed to the presence of reverted austenite. Homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling resulted in optimum tensile properties. A substantial increase in fusion zone toughness was observed after homogenisation+solutionising+aged condition due to a decrease in the content of austenite content compared to the directly aged condition. The reduction in microsegregation by diffusion of alloying elements from cell boundaries to the cell during homogenisation treatment is responsible for the decrease in austenite content.     

Ductility,structural quality,and fracture toughness of Al–Cu–Mg–Ag (A201) alloy castings

Abstract

The yield strength–elongation relationship in cast Al–Cu–Mg–Ag alloys (A201) castings has been investigated by analysing 11 datasets from the literature on premium quality castings. The data representing the elongation for a given yield strength have been found to fall within an envelope, the top limit of which represents the ductility potential of A201 alloy castings. A new quality index for A201 alloy castings is introduced. Moreover, using the fracture toughness equation by Hahn and Rosenfield, the estimated intrinsic fracture toughness values have been found to be consistent with the data reported in the literature.     

Influence of microstructure and chromium content on oxidation behaviour of spin cast high speed steels

Abstract

The influence of the chromium content and of the volume fraction of primary carbides on the thermal oxidation behaviour of spin cast high speed steels and semi-high speed steels used for the production of hot mill rolls was studied at 700°C. Oxidation nucleates at the carbide–matrix interface and carbides have a higher oxidation resistance than the matrix. Moreover carbides dissolve a higher amount of chromium than the matrix. As a consequence of these effects, the oxidation rate of these steels decreases by increasing the chromium content of the matrix and by decreasing the carbide volume fraction.