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.     

Predicting microstructural evolution and yield strength of microalloyed hot rolled steel plate

Abstract

A mathematical model has been developed to optimise process parameters for production of API grade steel plates by thermomechanical controlled processing at a plate mill in Bhilai Steel Plant, India. The model comprises the prediction of the microstructural evolution during hot rolling, the subsequent phase transformation, and, finally, the mechanical properties of microalloyed steels. Effects of chemistry and mill parameters on recrystallisation, grain growth, and precipitation kinetics were taken into consideration to describe the metallurgical processes. . The model has been validated through laboratory experiments as well as full-scale rolling at the plate mill.     

Ultrahigh strength hot rolled microalloyed steels: microstructural aspects of development

Abstract

Ultrahigh strength hot rolled microalloyed steels of yield strength 690 and 760 MPa with ferrite–bainite microstructure have been developed. Impact toughness of ～135 J at -40°C and a ductile–brittle transition temperature of less than approximately -70°C have been obtained in steels of gauge ～3 mm. The attractive strength–toughness combination was achieved by applying microalloying concepts and controlled rolling in an interactive manner. Alloy design aspects are qualitatively described in relation to the contributions of solute, grain size, precipitates, and microstructural constituents.     

Influence of hot rolling and heat treatment on structure and properties of HSLA steel explosively clad with austenitic stainless steel

Abstract

The present work aims at studying structure–property correlations in an explosively clad HSLA steel with austenitic stainless steel of AISI 304L grade. The clad plate was subjected to hot rolling followed by a quenching and tempering treatment to achieve better mechanical properties in the base plate. Optical microscopy studies revealed that the interface between the two steels was wavy in the as clad plate and the waviness decreased substantially due to hot rolling. Subsequent heat treatment has not shown any significant effect either. The base plate had tempered martensite/bainite structure in as clad or heat treated conditions and ferrite-pearlite-bainite structure in hot rolled condition. The grains were finer and elongated near the interface. The stainless steel exhibited equiaxed grain structure in as clad, hot rolled or heat treated plates. Tensile properties and charpy impact energy of the base plate were lowered due to hot rolling and then increased substantially due to heat treatment. The microhardness was observed to be a maximum at the bond interface for all three conditions studied. The shear bond strength was the highest in the as clad condition and decreased for the rolled as well as heat treated conditions. Scanning electron microscopy fractography on shear bond specimens revealed the presence of predominantly equiaxed dimples with few regions of rubbed fracture. Quantitative electron probe microanalysis across the bond interface indicated linear change in concentrations of nickel, chromium and manganese between the levels appropriate to the clad layer and base metal.     

Evolution of textures in hot rolled Nb - Ti and V - Nb microalloyed steels

Abstract

A considerable texture gradient in the through thickness direction was observed during hot rolling of Nb - Ti and V - Nb microalloyed steels. The most intense deformation texture for Nb- Ti steels was {113}〈110〉 at all depths, whereas for V - Nb steels the plane was shifted to {115}〈110〉 ; the angular difference between {113} and {115} is about a degree. The recrystallisation texture of austenite, {100}〈001〉 , transformed into {100} 〈011〉 component in the ferrite and indicated an increase in the intensity with increase in depth for both Nb - Ti and V - Nb steels. However, the intensity of this {100}〈011〉 texture was less for Nb - Ti steels compared to V - Nb steels at all depths. The reduced intensity of {100}〈011〉 texture in Nb -Ti steels is likely to be the reason for the superior formability and improved toughness of Nb - Ti steels as compared with V - Nb steels. The {100}〈011〉 type of texture has an undesirable effect on the edge formability of steels.     

In situ stir cast Al–TiB2 composite: processing and mechanical properties

Abstract

Elemental Ti and B powders of stoichiometric composition were mixed and added to molten aluminium. In situ TiB₂ particles were formed in the aluminium melt. On casting, an Al–TiB₂ composite was produced. Despite the presence of the Al₃Ti phase associated with the Al–Ti–B ternary system, the in situ TiB₂ particles, with sizes of 1–3 µm formed in the composite was able to yield an improvement of 57% increase in tensile strength, 66% in yield strength and 22% in modulus in an Al–15 vol.-%TiB₂ composite. The extent of improvement in these properties depended on the volume fraction in the composite. Fractography showed a texture of dimples seated with hexagonal TiB₂ particles indicating retention of high ductility in the composite, despite the fact that the predominance of the coarse Al₃Ti in the composite had led to premature rupturing.     

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.     

Characterisation of lead pipes used in the water industry: extrusion processing,alloy microstructure and their role in service failures

Abstract

The issue of leakage within the water distribution system is one of importance not only at an economic level for the industry, but also as a result of an environmental agenda addressing issues of water sustainability. The present work is concerned with leakage from lead based assets, in particular distribution pipes. Very little is known about the failure mechanisms within lead based assets. The present paper presents the findings from a study in which lead samples from intact and failed pipes, sourced from the Thames Water area, have been examined. The failure mechanisms have been identified at the macroscopic level and the pipe microstructure has been characterised – aspects of the microstructure control particular properties of the pipe (e.g. strength, creep and fatigue behaviour) and so may contribute to the potential failure modes. The present study is the first stage in a programme of work designed to develop a better understanding of the failure modes in lead assets, leading to the formulation of a more effective condition assessment model.     

Mechanical and corrosion properties of P/M-HIP super duplex stainless steel after different industrial heat treatments as used for large components

Abstract

Mechanical tensile and impact toughness tests and critical pitting corrosion temperature (CPT) tests were performed on samples of Duplok 27, a P/M-HIP duplex stainless steel containing copper, after heat treatments simulating industrial heat treatments of large components. It was shown that copper alloying has positive effects on mechanical tensile properties leading to hardening and more uniform deformation. No negative effects of copper alloying on corrosion resistance properties were found. A drastic drop in impact toughness values and CPT of samples cooled at controlled cooling rates is explained by the precipitation of intermetallic secondary phases or their precursors. Lower CPT of a NG-GTAW (narrow gap gas tungsten arc welding) welded joint is explained by the lower level of alloying than that of the base material. The high temperature region of precipitation of intermetallic secondary phases is shifted towards higher temperatures than assumed for Duplok 27 P/M-HIP duplex stainless steel.     

Transformation textures in new ultrahigh strength hot rolled microalloyed steel

Abstract

Texture development during thermomechanical processing of a newly developed ultrahigh strength microalloyed steel was investigated, with particular attention to through thickness texture gradient. A considerable texture gradient was evident, particularly in the 1/2 depth compared to the other three positions (surface, 1/8 depth, 1/4 depth). The most intense deformation texture at all depths was {113}〈110〉, which was also more prevalent at the centre than elsewhere, implying an overall dominance of {112}〈111〉 in the deformed austenite during hot rolling. The recrystallisation texture of austenite, {100}〈001〉, transformed into {100}〈011〉 component in the ferrite indicated an increase in intensity with increase in depth. The {100}〈011〉 type of texture has an undesirable effect on the delamination behaviour of steels. However, the centre and 1/4 depth contained considerable intensity of the desired texture, {332}〈113〉, which offsets the undesired {100}〈011〉 texture.     

Influence of boron on the microstructure and mechanical properties of Al-11.6Si-0.4Mg casting alloy modified with strontium

Abstract

The influence of the addition of Al-1B master alloy on the microstructure and mechanical properties of Al-11.6%Si-0.4%Mg alloy modified with 0.030%Sr has been investigated. The mechanical properties and fracture behaviour of the alloys as cast and after T6 heat treatment with three different melt treatments (no treatment; 0.030%Sr modifying treatment; and 0.030%Sr + 0.028%B combined melt treatment) were also compared. Al-1B master alloy has a strong action in refining the dendritic structure in near eutectic Al-Si casting alloys modified with Sr. The Sr+B combined melt treatment can improve considerably the mechanical properties of the alloys, both as cast and after T6 heat treatment. Fracture modes of the alloys with the Sr modifying treatment and the Sr+B combined melt treatment are typically ductile. However, fractographs indicate that the alloy with combined melt treatment suffered greater ductile deformation before fracture. The Sr+B combined melt treatment significantly improves the mechanical properties of near eutectic Al-Si casting alloys.     

Effects of Ca combined with Sr additions on microstructure and mechanical properties of AZ91D magnesium alloy

Abstract

Weight reduction to improve automobile fuel economy has triggered renewed interest in magnesium. The effects of Ca/Sr separate and composite additions to AZ91D magnesium alloy on its microstructure and mechanical properties have been investigated. The results indicate Ca can refine both the grain and eutectic phase of AZ91D magnesium alloy. Sr hampers microstructure refinement when composite Ca/Sr additions are made. In addition, separate Ca additions to AZ91D magnesium alloy increase yield strength but decrease elongation of this alloy. By adjusting the Ca/Sr composite proportions, additions to AZ91D magnesium alloy are able to improve both microstructure and mechanical properties of the alloy.     

Evaluation of hot strip mill backup roll cladding

Abstract

A new cladding material based on the Fe–Cr–Mo–V–C alloy system, suitable for submerged arc welding, has been designed for the refurbishment of forged and cast backup rolls used in the finishing stands of hot strip rolling mills. The work undertaken includes mechanical analysis, mechanical testing, and microstructural characterisation. The mechanical analysis indicated the nature and level of stresses operating near the surface of rolls; mechanical testing allowed material performance to be anticipated. An optimal post-weld heat treatment procedure, which maximises strength while minimising material strain hardening, was subsequently chosen. The microstructure of the candidate cladding material is a mixture of lower bainite and martensite, containing a very fine distribution of molybdenum carbides. In situresults have shown that welded rolls outperform traditional rolls, as the amount of steel rolled per millimetre of cladding material is 40% higher than with forged rolls and double that obtained with cast rolls.     

Mechanistic model for grain size and temperature dependence of flow stress in 316L austenitic stainless steel

Abstract

During plastic deformation of a polycrystalline material, both the grain interior and the grain boundary regions exhibit distinctly different dislocation behaviours at a given strain and temperature. Studying the variation of experimental flow stress with temperature, it seems that the flow stress of a fine grained polycrystalline material is mainly controlled by dislocation dynamics at and in the vicinity of grain boundaries. At low temperatures in a polycrystalline material, the dislocations are piled up at grain boundaries and the density of dislocations increases significantly in the grain boundary region, while at high temperatures the annihilation of dislocations take place at and in the vicinity of the grain boundaries during deformation. Therefore, the flow stress behaviour of a polycrystalline material can be understood in terms of the process of accumulation and annihilation of dislocations at and in the vicinity of grain boundaries at a given strain and temperature.     

Synthesis of boron nitride from ferroboron Part 1 – Nitriding

Abstract

Thermodynamic calculations and nitriding experiments have been carried out to convert ferroboron precursor to boron nitride in a mixture of NH₃ with N₂ gases under ambient pressure. The results show that complete nitridation can be achieved at 673 K with 20 vol.-%NH₃ in the gas mixture. The nitrided product consists of amorphous boron nitride, iron, and metastable iron nitrides. The measured weight gain follows a two stage, near parabolic kinetics accompanied by volume expansion, which causes spalling of the outer nitrided layer and facilitates further nitriding. An alternative reaction route using 100 vol.-%N₂ is also possible, but a higher reaction temperature (≥～1700 K) is needed. The product from the high temperature route contains partially crystallised (hexagonal) boron nitride, iron, and virtually no metastable iron nitride.     

Investigation of microstructure and mechanical properties of steel fibre–cast iron composites

Abstract

The aim of the present experimental study was to investigate improvement of the toughness and strength of grey cast iron by reinforcing with steel fibres. The carbon content of the steel fibres was chosen to be sufficiently low that graphite flakes behaving as cracks were removed by carbon diffusion from the cast iron to the steel fibres during the solidification and cooling stages. To produce a graphite free matrix, steel fibres with optimum carbon content were used and the reinforced composite structure was cast under controlled casting conditions and fibre orientation. Three point bend test specimens were manufactured from steel fibre reinforced and unreinforced flake graphite cast iron and then normalising heat treatments were applied to the specimens at temperatures of 800 and 850°C. The fracture toughness and strength properties of the steel fibre reinforced material were found to be much better than those of unreinforced cast iron. The microstructures of the composite at the fibre–matrix transition zone were examined.     

Dual phase versus TRIP strip steels: comparison of dynamic properties for automotive crash performance

Abstract

The need to simultaneously reduce vehicle emissions and increase the safety of passengers is encouraging the automotive industry to incorporate new technologies and materials into today's vehicles. To remain competitive, the steel industry has developed steel grades with increased energy absorbing properties allowing down gauging of body in white components to address the competition from alternative materials such as aluminium alloys and composites. Two of the more important developments are the introduction of dual phase (DP) and transformation induced plasticity (TRIP) grades for the automotive industry. These grades offer superior strength/formability and work hardening properties compared to conventional high strength grades of similar tensile strength. Utilising thinner gauge components with increased energy absorbing properties would permit addressing the mass/safety issues by the automotive industry. This paper relates the crash performance of a range of both commercial and experimental DP and TRIP grades. Dynamic tensile testing was performed at low and very high strain rates within the range of 0·001–200 s^?1, to allow an extensive analysis of the effect of strain rate on the material properties. Crash testing was also performed on closed top hat sections at low, medium and high strain rates and the results compared to the dynamically tested tensile specimens. This study helped clarify the enhanced performance offered by high strength DP and TRIP strip steel grades during dynamic tensile testing and impact loading conditions. This advantageous behaviour is attributed to the favourable microconstituents present in these novel grades and their deformation characteristics. This paper concentrates only on the crash properties measured from dynamic tensile tests. The microstructural analysis is presented in a separate publication.     

Synthesis and recyclability of a magnesium based composite using an innovative disintegrated melt deposition technique

Abstract

This study has addressed the feasibility of synthesising and recycling a silicon carbide reinforced magnesium composite using an innovative disintegrated melt deposition technique with the aim of improving the mechanical properties. Microstructural characterisation studies revealed a marginal decrease in porosity and reinforcement content, and no change in grain morphology, reinforcement distribution pattern, and interfacial integrity between matrix and reinforcement following recycling. Results of physical and mechanical property characterisation revealed increases in 0.2% yield strength, ultimate tensile strength, ductility, and coefficient of thermal expansion of the recycled specimens when compared with the parent composite. These properties have been rationalised in terms of the microstructural characteristics associated with the disintegrated melt deposited composite specimens. Particular emphasis was placed on studying the effect of recycling on the microstructure and properties of the composite.     

Silicon as grain refiner in niobium microalloyed hot rolled steel

The effect of silicon on grain refinement has been investigated in industrially produced Nb microalloyed steel. The synergistic effect of silicon in the presence of Nb is found to favour grain refinement in hot rolled strip, which has resulted in very fine ferrite grains of <3; μm. Evolution of such a fine grain has been related to the enhanced precipitation of Nb(CN) in the presence of silicon. The concept of effective niobium (Nb + Si) in such steels has been introduced.     

Microstructure evolution and mechanical properties of nickel based honeycomb sandwich

Abstract

A nickel based superalloy honeycomb sandwich was manufactured by high temperature brazing. The microstructure evolution and the out of plate mechanical properties were investigated for honeycomb sandwiches aged at 1000&degC in the present paper. The maximum tensile stress was 28·5 MPa and the compressive yield strength was 29·6 MPa for the original specimens. These parameters decreased to 22·9 and 20·5 MPa for specimens aged for 2 h, to 18·2 and 12·2 MPa for specimens aged for 5 h and to 20·2 and 14·3 MPa for specimens aged for 10 h respectively. With increasing aging time the tensile elongation decreased, the intermetallic compounds and the eutectic structure in the brazing region disappeared, and the solid solution approaching the matrix gradually increased.