Structure–property relationships in intercritical heat affected zone of low-carbon microalloyed steels

Abstract

The conditions for martensite formation in the intercritical heat affected zone (HAZ) of two low-carbon microalloyed steels have been investigated using optical and transmission electron microscopy. Based on Charpy V-notch testing of a large number of thermally cycled specimens, it is concluded that embrittlement within the intercritical HAZ of such steels is closely related to the development of twinned martensite during the weld cooling cycle. The reduced HAZ toughness probably arises from the associated stress concentrations developed in the surrounding ferrite matrix, which give rise to the initiation of brittle fracture in the ferrite.

MST/634     

Heat-affected zone oxidation properties in AISI 430 ferritic stainless steel welds

High temperature corrosion of ferritic stainless steel weldments has not previously been studied. This paper is an attempt to standardize a method for assessing the oxidation properties of the heat-affected zone (HAZ) of ferritic stainless steel welds. A weld thermal cycles simulator was used to model HAZ representative microstructure specimens, and the oxidation kinetics of both the base metal and selected HAZ specimens then studied by oxidizing the specimens at 950°C and 1 atmosphere in purified oxygen using a thermogravimetric rig. The oxidation mechanisms of both base metal and HAZ from representative specimens of the AISI 430 steel were studied using optical microscopy, scanning electron microscopy and energy dispersive X-ray analysis. The oxidation kinetics and morphology of both HAZ and base metal samples are reported.     

Microscopic deformation behaviour of martensitic–ferritic dual-phase steels

Abstract

The deformation behaviour of the two phases of three plain carbon dual–phase steels after various treatments has been studied using a scanning electron microscope equipped with a tensile straining stage. The distribution of strains between the ferrite and martensite phases, as well as among the different grains of each phase, was observed to be inhomogeneous. The martensite/ferrite strain ratio, which defines the degree of uniformity of straining between the phases, depends on the microstructural parameters of the steels: it increases with increasing volume fraction of martensite, but decreases as the carbon content of the martensite increases. Tempering at various temperatures causes a decrease in the martensite/ferrite microhardness ratio and hence causes an increase in the strain ratio. The macroscopic strain of the specimen at which the martensite begins to deform was also found to be dependent on the microstructural parameters. Regions of applicability of the existing theories of the strength of dual–phase steels can be estimated according to the deformation condition of the martensite.

MST/235     

Microstructural evolution and tensile behaviour of medium carbon microalloyed steel processed through two thermomechanical routes

Abstract

A multiphase microstructure was obtained in a medium carbon microalloyed steel using two step cooling (TSC) from a lower than usual finish forging/rolling temperature (800–850°C). A low temperature anneal was then used to optimise the tensile properties. A multiphase microstructure (ferrite–bainite–martensite) resulted from forging as well as rolling. These were characterised using optical and scanning and transmission electron microscopy. X-ray diffraction, transmission electron microscopy and hardness measurements were used for phase identification. Tensile properties and work hardening curves were obtained for both the forged and the rolled multiphase variants. A Jaoul–Crussard (J–C) analysis was carried out on the tensile data to understand the basic mode of deformation behaviour. Rolling followed by the TSC process produced a uniform microstructure with a very fine grain boundary allotriomorphic ferrite, in contrast to the forged variety, which contained in addition coarse idiomorphic ferrite. The volume fraction of ferrite and its contiguity ratio in the rolled microstructure were greater than in the forged grade. The rolled microstructure exhibited a better combination of strength and toughness than that of the forged material. The rolled steel work hardened more than the forged variety owing to its fine, uniform (bainite–martensite and ferrite) microstructure. Retained austenite present in these steels underwent a strain induced transformation to martensite during tensile deformation. The J–C analysis of the work hardening rates revealed typical three stage behaviour in both varieties during tensile deformation.     

Effect of post-weld heat treatment on heat affected zone microstructures of microalloyed C–Mn submerged arc welds

Abstract

The effect of various post-weld heat treatment (PWHT) cycles on the as welded heat affected zone (HAZ) microstructures of C–Mn steels microalloyed with niobium, or niobium plus vanadium, has been studied. Single pass welds were produced at an arc energy of 3·5 kJ mm^?1; examination was carried out using optical and electron microscopy, along with hardness and crack tip opening displacement testing. As welded, the C–Mn–Nb HAZ contained a significant proportion of auto tempered martensite. After PWHT at 550°C, isolated hard regions remained, but at 600°C all hard regions had been removed, with a concomitant increase in cleavage resistance. In contrast, ferrite with aligned second phase with lower hardness was found mainly in the as welded HAZ of the C–Mn–Nb–V steel. When the PWHT temperature was raised, HAZ hardness increased to a maximum at 600°C; overaging would be required to obtain improved toughness, although this would soften the parent plate. The results indicate that the current practice of specifying a common heat treatment procedure for steels to a given specification is not satisfactory; allowance should be made for the particular composition and as welded HAZ microstructure.

MST/1190     

Influence of processing parameters on lattice parameters in laser deposited tool alloy steel

Laser aided direct metal deposition (DMD) has been used to form AISI 4340 steel coating on the AISI 4140 steel substrate. The microstructural property of the DMD coating was analyzed by means of scanning electron microscopy, transmission electron microscopy and X-ray diffractometry. Microhardness of the DMD was measured with a Vickers microhardness tester. Results indicate that DMD can be used to form dense AISI 4340 steel coatings on AISI 4140 steel substrate. The DMD coating is mainly composed of martensite and retained austenite. Consecutive thermal cycles have a remarkable effect on the microstructure of the plan view of the DMD coating and on the corresponding microhardness distribution. Orientation relationships among austenite, martensite and cementite in the DMD coating followed the ones in conventional heat treated steels. As the laser specific energy decreased, cooling rate increased, and martensite peaks broadened and shifted to a lower Bragg's angle. Also martensite lattice parameters increased and austenite lattice parameters decreased due to the above parameter change.     

Correlation between microstructure and creep performance of martensitic/austenitic transition weldment in dependence of its post-weld heat treatment

This paper deals with the influence of post-weld heat treatment (PWHT) of T92/TP316H martensitic/austenitic transition weldment on the resulting microstructure and creep characteristics. Experimental weldments were fabricated by gas tungsten arc welding using a nickel-based weld metal (Ni WM). After the welding, two individual series of produced weldments were heat-treated according to two different PWHT procedures. The first “conventional PWHT” was carried out via subcritical tempering (i.e. bellow Ac₁ temperature of T92 steel), whereas the other one, the so-called “full PWHT” consisted of a complete reaustenitization of the weldments followed by water-quenching and final tempering. The use of “conventional PWHT” preserved microstructural gradient of T92 steel heat-affected zone (HAZ), consisting of its typical coarse-grained and fine-grained subregions with tempered martensitic and recrystallized ferritic–carbidic microstructures respectively. In contrast, the “full PWHT” led to the complete elimination of the original HAZ via transformation processes involved, i.e. the reaustenitization and back on-cooling martensite formation. The observed microstructural changes depending on the initial PWHT conditions were further manifested by corresponding differences in the weldments’ creep performance and their failure mode. The weldments in “conventional PWHT” state ruptured after long-term creep tests by premature “type IV failure” within their recrystallized intercritical HAZs. On the contrary, the long-term creep behavior of the weldments processed by “full PWHT” was characterized by their remarkable creep life extension but also by the occurrence of unfavorable “decohesion failure” along T92/Ni WM interface.     

Microstructural evolution of simulated heat-affected zone in modified 2.25Cr-1Mo steel during high temperature exposure

The effect of heat input with 20, 50 and 80 kJ/cm on the microstructural evolution of the simulated heat affected zone (HAZ) has been studied in a modified 2.25Cr-1Mo steel. The microstructures of simulated coarse-grained HAZ has been examined by optical metallography and transmission electron microscopy. It was found that large amounts of martensite with small quantities of bainite exist in the specimen with 20 kJ/cm. However, significant amount of bainite with a few amounts of allotriomorphic ferrite can be detected in the specimen with 50 kJ/cm heat input. In the case of heat input with 80 kJ/cm, the simulated HAZ microstructures were composed chiefly of bainite with a few amounts of martensite and allotriomorphic ferrite. In order to study the sequences of carbide transformation, the HAZ specimens were tempered at 700°C for different intervals (1, 2, 5, 10, 20, 50, 100, 200 and 500 h). The sequence of carbide transformation in the HAZ zone of this modified 2.25Cr-1Mo steel has been proposed.     

Effect of silicon content on microstructure and toughness of simulated heat affected zone in titanium killed steels

Abstract

hree steels having different silicon contents were prepared to study the microstructure and toughness of the thermally simulated heat affected zone (HAZ) in titanium killed steels. For a low silicon addition, the oxygen content in the molten steels decreased remarkably. This in turn caused a change in the inclusion phase from predominantly titanium oxide to titanium nitride (TiN), the change being accompanied by two major microstructural modifications. The austenite grain size became refined and the quantity of intragranularly nucleated acicular ferrite decreased. The microstructural change was found to cause coarsening of Charpy fracture surfaces and deterioration of HAZ toughness of the steels. The minor change of silicon content therefore has a profound influence on the properties of titanium killed steels.

MST/1503     

Effect of epitaxial ferrite on yielding and plastic flow in dual phase steel in tension and compression

Abstract

A low carbon, microalloyed steel was heat treated to obtain dual phase microstructures containing constant levels of 18 and 25 vol.-% martensite at two levels of microstructural refinement and with varying epitaxial ferrite content. Tensile and compression tests were conducted at a strain sensitivity of 2 × 10^-5. Elastic limits in tension and compression were indistinguishable and very low, suggesting that mobile dislocations were present in the ferrite as a consequence of stress relaxation processes. These mobile dislocations accommodated the volume increase accompanying the austenite to martensite transformation during heat treatment. Epitaxial ferrite had little effect on the 0·2% proof stress, but average proof stresses were generally higher in compression than in tension owing to residual stresses in the martensite and ferrite following heat treatment. The residual stresses calculated from this asymmetry in the proof stresses were small because of stress relaxation in the ferrite at the temperature at which the martensite formed. Epitaxial ferrite significantly increased uniform elongation in tension with a small decrease in tensile strength for both levels of martensite in the finer microstructure but only at the 18 vol.-% martensite level in the coarser microstructure. The cause of the increased ductility was the effect of epitaxial ferrite on the work hardening rate between approximately 0·5 and 3% strain; epitaxial ferrite reduced the work hardening rate in this range of strain.     

Role of pulsed current on metallurgical and mechanical properties of dissimilar metal gas tungsten arc welding of maraging steel to low alloy steel

This research work encompasses the investigations carried out on the mechanical and metallurgical properties of maraging steel and AISI 4340 aeronautical steel weldments. The materials were joined by continuous current gas tungsten arc welding (CCGTA) and pulse current (PCGTA) gas tungsten arc welding processes using ErNiCrMo-3 filler wire. Cross sectional macrostructures confirmed proper deposition of the fillers and lack of discontinuities. Optical microscopy studies revealed that at the maraging steel–weld interface, martensite in distorted and block forms prevailed in CCGTA and PCGTA weldments whereas tempered martensite was predominant at the low alloy–weld interfaces of both the welds. Scanning electron microscopy (SEM) with energy dispersive analysis of X-rays (EDAX) analysis apparently showed less elemental migration in PCGTA weldments as compared to the other. Results of X-ray diffraction analysis recorded possible phase formations in various zones of the weldments. Microhardness profiles in either weld zones followed a constant trend whereas it showed a downtrend in the heat affected zones (HAZ) of the maraging steel and very high hardness profiles were observed in the low alloy steel side. Tensile studies on various factors and impact testing showed that PCGTA weldments outperformed the continuous ones in terms of strength, ductility and toughness. Fractograph analysis depicted the nature of failures of tensile and impact tested specimens. Comparison analyses involving influence and nature of pulsed current welds over continuous ones were done to determine the possibility of implementing these joining processes in aerospace applications. Weldments fabricated using PCGTA technique proved to be superior to the other, resulting in exceptional mechanical properties.     

Effect of chromium content on the microstructure and mechanical properties of multipass MMA,low alloy steel weld metal

Effect of chromium content in the range of 0.05–0.91 wt% on the microstructure and mechanical properties of Cr–Ni–Cu low alloy steel weld metal was investigated. All welds were prepared by manual metal arc welding technique in flat position. Microstructure of the welds was examined by optical and scanning electron microscope in both columnar and reheated regions of the weld metal. The results showed increase in acicular ferrite and microphases formed at the expense of primary ferrite and ferrite with second phase with steady refinement of microstructure. According to these microstructural changes, yield and ultimate tensile stresses, Hardness and Charpy V-Notch impact toughness increased, whereas elongation decreased. Increase in Charpy impact value is thought to be due to fine dispersed spheroidized dark microphases at high chromium contents.     

A microstructural study of local melting of grey cast iron by a static plasma arc

The microstructural changes produced by plasma arc local-melting method in the fusion region or deposited metal, fusion boundary region and heat-affected zone (HAZ) of flake graphite cast iron were investigated. Cylindrical base metal specimens were locally melted at fixed time intervals under a stationary plasma torch using argon plasma gas and Ar + 10% H₂ shielding gas. The welds were produced autogenously and with filler metals. The cooling rate in the fusion region was recorded. Evaluation of the fusion boundary area included metallurgical analysis, microhardness and electron probe microanalysis. In the absence of filler metal, the structure of fusion region where completely melted was ledeburite. In the centre of the fusion region the structure appears to be that of hypoeutectic white cast iron, and in the fusion boundary of this region the structure appears to be fined ledeburite. In the fusion boundary region where supercooled phenomena can occur, fine martensite precipitates appear along the fusion line for small heat input and secondary graphite is seen for large heat input. The HAZ is composed of white martensitic, dark martensitic and martensitic-fine pearlitic layers for small heat input, and of finely laminated pearlitic and finely laminated pearlitic-ferritic layers for large heat input. Using filler metal, the structure of the deposited metal is found to be that of a nickel austenitic matrix precipitating tiny graphite nodules or slim graphite for nickel and Ni-Fe filler metals, but to be a pearlitic matrix for iron filler metal. In the fusion boundary region, nickel-martensite, eutectic or slim graphite and fine nickel-martensite are precipitated from the nickel system filler metal, and the columnar structure of ferrite and pearlite is obtained from iron filler metal. The HAZ is composed of thin ledeburitic, martensitic, martensitic-fine pearlitic and finely laminated pearlitic layers for the nickel filler metal, of ledeburitic and finely laminated pearlitic layers for the Ni-Fe filler metal, and of thick ledeburitic, eutectic graphite-crystallized and finely laminated pearlitic layers for iron filler metal. The hardness of the ledeburitic layer and the nickel-martensitic portion is very high where the liquid existed. The diffusion of nickel from the deposited metal into the HAZ can occur at least until the fused base metal of HAZ.     

Application of single crystals to achieve quantitative understanding of weld microstructures

Abstract

The inter-relationship between the weld pool shape and the weld microstructure is a critical factor that determines the physical integrity and other important properties offusion welds. In the present work, large single crystals of an Fe–15Ni–15Cr alloy have been used to increase basic understanding of the factors that influence the development of weld microstructures. Oriented ternary alloy single crystals were used to make electron beam welds along various principal directions lying in different principal crystallographic planes. Using oriented single crystals it was possible to obtain crucial microstructural information that is ordinarily lost when welds are made on normal polycrystalline specimens. This quantitative information regarding the microstructural properties of electron beam welds has provided valuable new insight into the fundamentals of the relationships between weld pool shapes and weld microstructures. A new three-dimensional geometrical analytical method has been developed to interpret the microstructural information resulting from welds made using oriented single crystals. This analytical method establishes a direct correlation between the three-dimensional weld pool shape and the dendritic microstructures that are observed in two-dimensional transverse micrographs, and can be used to reconstruct the three-dimensional weld pool shape. Single crystal multipass and single pass bicrystal welds have also been examined. Overlapping multipass single crystal welds showed remarkable reproducibility from pass to pass and replicated the microstructural patterns observed in single pass welds. The microstructure of butt welds joining two single crystals with different orientations showed a one to one correspondence with that associated with each individual crystallographic orientation, and the microstructure essentially represented a composite of two single pass microstructures corresponding to the individual crystal orientations.

MST/3166     

Phase transformations in low-alloy steel laser deposits

We examined the microstructure evolution in medium-carbon low-alloy steel upon laser engineering net shape (LENS) (LENS is a trademark of Sandia National Laboratories and the US Department of Energy, Albuquerque, NM). Involved was the deposition of 14 superimposed fine layers. Several characterization techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), nanoindentation and electron back scattered diffraction (EBSD) were used in conjunction with three-dimensional finite element thermal modeling to rationalize the transformation mechanisms. Delta ferrite was the primary phase to solidify from the melt. Solid-state austenitisation led to allotriomorphic hexagonal prisms that grew following a unique direction depending upon the parent delta-grain crystallographic orientation. A supersaturated lower bainitic plate was the main phase to have transformed from austenite, except for the first two deposited layers where martensite predominated. The supersaturated plate underwent a sudden-tempering reaction at the 10th layer but was confined at the plate boundaries. This tempering reaction became more transgranular and increasingly affected retained austenite and microphases for the underlying layers. Coalescence of carbon-depleted ferrite plates gave rise to the steady-stage microstructure at the fourth layer. This complicated microstructural evolution corroborated the microhardness fluctuations through all deposited layers.     

Microstructural evolution in bainite,martensite, and δ ferrite of low activation Cr–2W ferritic steels

Abstract

The microstructural evolution in (2–15)Cr–2W–0·1C (wt-%) firritic steels after quenching, tempering, and subsequent prolonged aging was investigated, using mainly transmission electron microscopy. The steels examined were low induced radioactivation ferritic steels for fusion reactor structures. With increasing Cr concentration, the matrix phase changed from bainite to martensite and a dual phase of martensite and δ ferrite. During tempering, homogeneous precipitation of fine W₂C rich carbides occurred in bainite and martensite, causing secondary hardening between 673 and 823 K. With increasing tempering temperature, dislocation density decreased and carbides had a tendency to precipitate preferentially along interfaces such as bainite or martensite subgrain boundaries. During aging at high temperature, carbides increased in size and carbide reaction from W₂C and M₆C to stable M₂₃C₆ occurred. No carbide formed in δ ferrite. The precipitation sequence of carbides was analogous to that in conventional Cr–Mo steels.

MST/1049     

Microstructure evolution in heat affected zone of T4003 ferritic stainless steel

In this work, microstructural characteristics and development within the heat affected zone (HAZ) of T4003 ferritic stainless steel (FSS) welded joint were investigated combining experimental measurement with finite element simulation of welding temperature field. The results indicate that the HAZ was characterized with heterogeneous microstructure due to the extensive peak temperature range which could be divided into three sub-zones named as HAZ1, HAZ2 and HAZ3. The HAZ1 (the region next to weld zone boundary) experienced peak temperatures of 1300–1500 °C during welding process. This region presented almost fully δ ferrite microstructure with irregular grain, which was attributed to the high element diffusion rate and the absence of elevated-temperature austenite. The HAZ2 (center region of HAZ) suffered the peak temperatures of 1150–1300 °C. It presented martensite + δ ferrite dual microstructure with limited grain growth due to the formation of γ phase at grain boundaries. The HAZ3 (the region closed to the base metal) was undergone the peak temperatures of 830–1150 °C and was characterized with both martensite and ferrite structure.     

Effect of electron beam welding on the microstructures and mechanical properties of thick TC4-DT alloy

Electron beam welding (EBW) was applied to 50 mm thick damage-tolerant Ti–6Al–4V (TC4-DT) alloy, and microstructure, microhardness and tensile properties of the defect-free welded joints were examined. The results indicated that the microstructure of the base metal is composed of primary α phases and the lamellar (α + β) bimodal structure. For the EBW joint, martensite basketweave microstructure is formed in fusion zone (FZ). Moreover, the heat affected zone (HAZ) near FZ consists of acicular martensite and a small portion of primary α phase. The HAZ near base metal consists of primary α phase and transformed β containing aciculate α. It is found that the boundary of the two portions of the HAZ was dependent on the β phase transus temperature during weld cooling. Microhardness values for FZ and HAZ are higher than that of base metal, and there are the peak values for the HAZ near the weld metal. The fracture locations of all the EBW tensile specimens are in base metal, and the ultimate tensile strength of the joints may reach about 95% of the base metal. In addition, with the depth increasing along the weld thick direction, the grain size of the FZ decreases and microhardness increases.     

Nature of inclusions in steel weld metals and their influence on formation of acicular ferrite

Abstract

The chemistry and structure of weld metal inclusions has been studied. Four submerged arc welds which utilized plate and consumables to cover a range of oxygen and deoxidant contents were examined. Analysis of the inclusions was carried out on carbon extraction replicas in a Philips 400T scanning transmission electron microscope, fitted with an energy dispersive analyser. Two major types of inclusion were found. With weld metal aluminium approaching the stoichiometric ratio with oxygen, the inclusions were crystalline and had a spinel structure at the centre with a discontinuous, polycrystalline, titanium-rich phase on the surface. With weld metal oxygen high compared with the stoichiometric ratio with aluminium, inclusions were glassy and essentially manganese silicate in composition, again with areas of a polycrystalline, titanium-rich phase on the surface. The interinclusion spacing varied little with weld metal oxygen content in the range 0·0268–0·0858 wt-%. The spacing was found to be of a similar order to the acicular ferrite grain size. The titanium-rich surface phase in all the welds was of fcc structure with a lattice parameter of 0·42 nm, which suggests a mixture of TiO and TiN, possibly rich in TiO. The spinel phase was also fcc and had a composition between galaxite (Al₂O₃MnO) and γ-alumina. Both these phases have a low lattice misfit with ferrite. A low lattice misfit of the inclusion surface layers with ferrite coupled with closely spaced inclusions would seem to be key factors in the development of an acicular ferrite weld metal microstructure.

MST/543     

Effect of aluminium concentration in filler alloys on HAZ cracking in TIG welded cast Inconel 738LC superalloy

Abstract

The effect of concentration of aluminium in filler alloys on heat affected zone (HAZ) microfissuring in TIG welded cast Inconel 738LC (IN 738LC) superalloy was studied. Three fillers, IN 625, IN 617 and Haynes 214, with aluminium concentrations varying from 0·2 to 4·5 wt-%, respectively, were used to TIG weld cast IN 738LC alloy plates subjected to two different preweld heat treatments. One preweld heat treatment was the standard solution heat treatment at 1120°C for 2 h followed by argon quenching. The second was a novel overaging treatment developed by the present authors, termed UM treatment, involving solution treatment at 1120°C followed by air cooling and subsequent ageing at 1025°C followed by water quenching. Detailed microstructural analysis of the welds and base metal was done by optical and analytical electron microscopy. Intergranular microfissures were observed in the HAZ of all the welds, irrespective of the filler alloy and the preweld heat treatment, while no cracks were observed in the fusion zone in any of the samples. The cracks were mostly found to be associated with constitutionally liquated MC carbides, borides, sulphocarbides, γ–γ′ eutectic and γ′ precipitates. The cracking was found to increase with increase in the fusion zone hardness and the aluminium content of the fillers, i.e. it was minimum for IN 625 and maximum for Haynes 214, for a particular preweld heat treatment. Between the two heat treatments, the UM treated samples with a smaller base metal hardness, however, exhibited a considerably reduced HAZ cracking. That is, the hardness of the fusion zone as well as the base metal appears to have a significant effect on the cracking susceptibility of the welds made with different fillers.