Microstructure Of Heat Affected Zone Research Articles

Hydrogen evolution of Cu precipitation 780 MPa grade steel HAZ with consideration of its cold cracking susceptibility

The Cu precipitation 780 MPa grade steel has higher resistance to cold cracking than the conventional steels. To study this, we have investigated the hydrogen evolution characteristics of the Cu steel heat-affected zone (HAZ) by the thermal desorption spectroscopy method and compared with those of conventional 780 MPa steel HAZ. The thermal cycle tests were conducted to obtain the HAZ microstructures, and the peak temperatures were selected from 500 to 1400 °C at intervals of 100 °C. Hydrogen was then charged, and the hydrogen evolution curves were measured at a heating rate of 100 °C/h. The results show that the amounts of hydrogen released from the specimens whose peak temperatures of the thermal cycle tests were 1000 °C or higher are lower, while the results of the conventional steels have the opposite tendency. We numerically calculated the hydrogen evolution curves to determine physical constants, and using these constants, we also calculated the hydrogen concentration profiles of the y-groove weld cracking test and found that the hydrogen concentration of the Cu steel HAZ near the fusion line tends to become lower than those of the conventional steel HAZ. This phenomenon is considered one of the reasons for the higher cold cracking resistance of the Cu steel.

Microstructure–properties relationships in martensitic stainless steel resistance spot welds

The paper addresses the phase transformations and mechanical response of martensitic stainless steel resistance spot welds. The fusion zone microstructure consists of carbon-rich martensite together with a relatively high amount of retained delta ferrite along the grain boundaries with a transition in solidification mode from equiaxed to columnar dendritic grains across the fusion zone. The heat affected zone microstructure is featured by martensitic matrix together with carbide precipitation. The very high hardness of the fusion zone and the heat affected zone, the sharpness of the notch at sheet/sheet interface, which is located in the hard microstructural zone, and the presence of delta ferrite in the weld nugget play important roles in failure characteristics and mechanical performance of the joint.

On the development of a new pre-weld thermal treatment procedure for preventing heat-affected zone (HAZ) liquation cracking in nickel-base IN 738 superalloy

Hot cracking in the heat-affected zone (HAZ) of precipitation strengthened nickel-base superalloys, such as IN 738, during fusion welding remains a major factor limiting reparability of nickel-base gas turbine components. The problem of HAZ intergranular cracking can be addressed by modifying the microstructure of the pre-weld material through thermal treatment, which requires significant understanding of the critical factors controlling cracking behaviour. The decomposition of Mo-Cr-W-and Cr-rich borides in the alloy, among other factors, has been observed to contribute significantly to non-equilibrium intergranular liquation and, hence, intergranular liquation cracking during welding. Gleeble physical simulation of HAZ microstructure has also shown that non-equilibrium liquation is more severe in the vicinity of decomposed borides in the alloy and can occur at temperatures as low as 1,150 °C. Although currently existing pre-weld heat treatments for IN 738 superalloy minimize the contributions of dissolution of second phases, including borides, to HAZ intergranular liquation, these heat treatments are not industrially feasible due to process-related difficulties. Therefore, a new industrially feasible and effective pre-weld thermal treatment process, designated as FUMT, was developed during the present research by controlling both the formation of borides and the segregation of boron at the grain boundaries in the pre-weld heat-treated material. This thermal treatment was observed to very significantly reduce intergranular HAZ cracking in welded IN 738 superalloy. The details of the development process and developed procedure are presented in this paper.

Resilience and ductility of Oxy-fuel HAZ cut

Cutting processes affect the material to a deeper or shallower attached-to-the-cut zone. Its microstructure, its hardness and mechanical properties are changed. Also the cutting process introduces surface roughness and residual stresses. In most cases it is recommended to remove this region by grinding, in order to keep a smoother surface, free from the above mentioned effects. This work presents the characterization results of the Heat Affected Zone (HAZ) of a steel plate of grade S460M, with a thickness of 25 mm, cut by flame oxyfuel gas cutting. The HAZ microstructure is observed (and the depth of the HAZ measured), the hardness profile and the stress vs. strain curves until fracture are measured by testing micro-tensile samples, instrumented with strain gauges. Micro-Tensile specimens are 200 microns in thickness and were obtained from layers of the HAZ at different distances from the oxy-fuel cut. The obtained stress-strain curves are compared with the hardness measurements and the observed metallography.

Effect of Boron on CGHAZ Microstructure and Toughness of High Strength Low Alloy Steels

Effect of boron on the microstructure and impact toughness in the coarse-grained heat-affected zone (CGHAZ) of two high strength low alloy steels, boron-free and boron-containing, was investigated by means of weld thermal simulation test. The result shows that, for the boron-free steel, a microstructure consisting of grain boundary ferrite degenerates pearlite and granular bainite for longer t8/5 (the cooling time from 800 to 500 °C), while lath bainite for shorter t8/5. For the boron-containing steel, granular bainite is dominant for a wide range of t8/5. Continuous cooling transformation (CCT) study on the CGHAZ indicates that the transformation start temperature decreases by about 50—100 °C under different t8/5, for the boron-containing steel compared with the boron-free steel. The presence of boron suppresses the nucleation of ferrite at prior austenite grain boundaries and hence enlarges the range of t8/5 for granular bainite transformation. However, the addition of boron deteriorates the impact toughness of CGHAZ, which may be due to a markedly increased fraction of martensite-austenite (M-A) constituents and decreased fraction of high angle grain boundaries.

Microstructure characteristics and temperature-dependent high cycle fatigue behavior of advanced 9% Cr/CrMoV dissimilarly welded joint

Advanced 9% Cr and CrMoV steels chosen as candidate materials are first welded by narrow-gap submerged arc welding (NG-SAW) to fabricate the heavy section rotor. The present work focuses on studying the high-cycle fatigue (HCF) behavior of advanced 9% Cr/CrMoV dissimilarly welded joint at different temperatures. Conditional fatigue strength of this dissimilarly welded joint was obtained by HCF tests at room temperature (RT), 400°C and 470°C. It was observed that the failure occurred at the side of CrMoV base metal (BM), weld metal (WM) and heat affected zone (HAZ) of CrMoV side over 5×107 cycles for the specimens tested at RT, 400°C and 470°C. The detailed microstructures of BMs, WMs and HAZs as well as fracture appearance were observed by optical microscopy (OM) and scanning electron microscopy (SEM). Precipitation and aggregation of carbides along the grain boundaries were clearly detected with the increase of temperature, which brought a negative effect on the fatigue properties. It is interesting to note that the inclusion size leading to crack initiation became smaller for the HCF test at higher temperature. Therefore, reduction in the inclusion size in a welded joint helps to improve the HCF performance at high temperature.

Microstructure and Toughness of Simulated Heat-Affected Zone of Laser Welded Joint for 960 MPa Grade High Strength Steel

The microstructure and toughness of coarse grain zone (CGZ) and mixed grain zone (MGZ) for laser welded 960 MPa grade high strength steel joints were investigated by thermal simulation with a Gleeble-3500 thermal simulator. The results show that microstructure of the stimulated CGZ mainly consists of uniform interweaved lath martensite, and grain growth is not severe upon increasing the cooling time (t 8/5). Microstructure of the stimulated MGZ presents strip-like in low peak temperature, and small block martensite is formed on the grain boundary. However, in high peak temperature, the strip-like microstructure disappears and small block martensite presents net-like structure. The lath character for MGZ and CGZ is very obvious under TEM observation, and the average lath thickness of BM, MGZ, and CGZ is 100, 150 and 200 nm, respectively. The impact energy and microhardness of CGZ are higher than MGZ and reduce with increasing the cooling time. The fracture toughness deteriorating drastically for MGZ may be related with the formation of the mixture microstructure, in which the small block martensite is distributed in the shape of a network.

The influence of various factors on the geometric profile of laser lap welded T-joints

The effect of processing parameters, assembly parameters, and plate thickness on the weld bead geometry was examined, and the microstructure and microhardness of the typical joints were investigated in this paper. There are two kinds of welds, these are the Y-type and V-type welds in different heat input. Heat input has a small effect on bead width at the interface when the thickness of face plate is more than 3 mm. The variation of bead geometry is little when gap and offset size is small, while porosity or depression forms with increasing gap and offset. Plate thickness exerts a remarkable effect on the bead width at interface, and a big bead width at interface can be achieved by adopting thin face plate. The centricity of the laser beam is a big problem in laser lap welding T-joints with thin core plate, while bead width at interface is small in laser lap welding T-joints with thick face plate. Microstructure of heat-affected zone and fusion zone is composed of lath martensite and bainitic ferrite. Microhardness of fusion zone is lowest due to the coarse lath martensite. The desired bead geometry of laser lap welded T-joints can be produced by adopting low welding speed, thin face plate, and small gap.

Austenite grain growth and microstructure control in simulated heat affected zones of microalloyed HSLA steel

The roles of microalloying niobium, titanium and vanadium for controlling austenite grain growth, microstructure evolution and hardness were investigated at different simulated heat affected zones (HAZ) for high strength low alloy (HSLA) S690QL steel. High resolution FEG-SEM has been used to characterize fine bainitic ferrite, martensite and nanosized second phases at simulated coarse and fine grain HAZs. It was found that for Ti bearing steel (Ti/N ratio is 2) austenite grain had the slowest growth rate due to the presence of most stable TiN. The fine cuboidal particles promoted intragranular acicular ferrite (IGF) formation. Nb bearing steel exhibited relatively weaker grain growth retardation compared with titanium bearing steels and a mixed microstructure of bainite and martensite was present for all simulated HAZs. IGF existed at coarse grain HAZ of Ti+V bearing steel but it was totally replaced by bainite at fine grain HAZs. Hardness result was closely related to the morphology of bainitic ferrite, intragranular ferrite and second phases within ferrite. The microstructure and hardness results of different simulated HAZs were in good agreement with welded experimental results.

Effect of Laser Welding Process Parameters on Microstructure and Mechanical Properties on Butt Joint of New Hot-Rolled Nano-scale Precipitation-Strengthened Steel

A 4 kW fiber laser was chosen to weld the new hot-rolled nano-scale precipitation-strengthened steel with a thickness of 4.5 mm. The effect of laser power, defocusing distance, and welding speed on the welded joint appearance was examined, and the microstructure and mechanical properties on the typical butt joints were investigated. Results showed that increasing laser welding power may cause faster downward flow of molten metal to produce greater root humping. With the welding speed increasing, the average welding seam (WS) width decreased, and the average WS and heat-affected zone (HAZ) hardness increased. The microstructures of WS, fusion line, and coarse grain heat-affected zone were lath martensite, but the growth direction of the original austenite grain boundaries was significantly different. The microstructures of fine grain heat-affected zone were ferrite and martensite, and the microstructure of mixed grain heat-affected zone contained ferrite, massive M/A island, and a small amount of martensite. The micro-hardness values of WS, HAZ, and base metal (BM) were 358, 302, and 265 HV, respectively. The butt joint fracture at the BM far from the WS and the welded joint tensile strength are observed to follow proportional relationship with hardness.

Study on Microstructure and Property in Heat-Affected Zone of X80 Pipeline Steel

Experimental methods, such as OM, TEM and property testing, were used to study the HAZ microstructure and property of X80 pipeline steel. HAZ can be divided into three regions, such as coarse grain zone, transition zone and fine grain zone. Granular bainite of CGHAZ consists of bainite ferrite (BF) lath and M/A constituents. Charpy impact energy of the HAZ is much lower than that of base metal, for the grain size of CGHAZ is much larger than that of BM, and crack formation and propagation of granular bainite is easy.

Microstructural Characterization of Thermomechanical and Heat-Affected Zones of an Inertia Friction Welded Astroloy

The behaviour of γ’ phase to thermal and mechanical effects during rapid heating of Astroloy, a powder metallurgy nickel-based superalloy has been investigated. The thermo-mechanical-affected zone (TMAZ) and heat-affected zone (HAZ) microstructures of an inertia friction welded (IFW) Astroloy were simulated using a Gleeble thermo-mechanical simulation system. Detailed microstructural examination of the simulated TMAZ and HAZ and those present in actual IFW specimens showed that γ’ particles persisted during rapid heating up to a temperature where the formation of liquid is thermodynamically favored and subsequently re-solidified eutectically. The result obtained showed that forging during the thermo-mechanical simulation significantly enhanced resistance to weld liquation cracking of the alloy. This is attributable to strain-induced rapid isothermal dissolution of the constitutional liquation products within 150 μm from the center of the forged sample. This was not observed in purely thermally simulated samples. The microstructure within the TMAZ of the as-welded alloy is similar to the microstructure in the forged Gleeble specimens.

On the theory of CO2 corrosion reactions – Investigating their interrelation with the corrosion products and API-X100 steel microstructure

This electrochemical research investigates the interrelation between the corrosion reactions, the growth and characteristics of the corrosion products, and the microstructures of simulated Heat-Affected Zones (HAZs) from API-X100 steel. In saline CO2-saturated solutions, the corrosion products suppress more the anodic dissolution than the cathodic reactions. The corrosion products of ferritic microstructures are thicker and of higher cathodic currents than those of acicular-ferritic and martensitic microstructures, which are intrinsically less reactive. The significances of chloride, iron oxides, and Fe3C with the growth and reactivity of the corrosion products were explored. Schematics depicting the open-circuit reactions were made from electrochemical impedance spectroscopy measurements.

Effects of Oxides on Tensile and Charpy Impact Properties and Fracture Toughness in Heat Affected Zones of Oxide-Containing API X80 Linepipe Steels

This study is concerned with effects of complex oxides on acicular ferrite (AF) formation, tensile and Charpy impact properties, and fracture toughness in heat affected zones (HAZs) of oxide-containing API X80 linepipe steels. Three steels were fabricated by adding Mg and O2 to form oxides, and various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs. The no. of oxides increased with increasing amount of Mg and O2, while the volume fraction of AF present in the steel HAZs increased with increasing the no. of oxides. The strengths of the HAZ specimens were generally higher than those of the base metals because of the formation of hard microstructures of bainitic ferrite and granular bainite. When the total Charpy absorbed energy was divided into the fracture initiation and propagation energies, the fracture initiation energy was maintained constant at about 75 J at room temperature, irrespective of volume fraction of AF. The fracture propagation energy rapidly increased from 75 to 150 J and saturated when the volume fraction of AF exceeded 30 pct. At 253 K (−20 °C), the total absorbed energy increased with increasing volume fraction of AF, as the cleavage fracture was changed to the ductile fracture when the volume fraction of AF exceeded 45 pct. Thus, 45 vol pct of AF at least was needed to improve the Charpy impact energy, which could be achieved by forming a no. of oxides. The fracture toughness increased with increasing the no. of oxides because of the increased volume fraction of AF formed around oxides. The fracture toughness did not show a visible correlation with the Charpy absorbed energy at room temperature, because toughness properties obtained from these two toughness testing methods had different significations in view of fracture mechanics.

Effect of Welding Heat Input on Microstructure and Mechanical Properties of HSLA Steel Joint

Gas tungsten arc welding of HSLA steel was conducted with different welding heat inputs, and the influences of welding heat input on the microstructure, Vickers hardness, and impact toughness of heat-affected zone (HAZ) in prepared joints were systematically investigated. The microstructure in HAZ with low welding heat input mainly consisted of martensite, and the microhardness of coarse grain HAZ was measured higher than that of fine grain zone. The results show that increasing the welding heat input could suppress the formation of martensite and reduce the microhardness of HAZ. However, the impact toughness of HAZ was not monotonously improved with the increase of welding heat input. It is deemed that the coarsened grain, the formation of upper bainite, and the non-uniform distribution of carbides and inclusions in HAZ could degrade the impact toughness. Tests demonstrate that the optimum comprehensive properties of HAZ were obtained when the welding heat input was 0.67 kJ/mm.

The Effect of Welding Conditions on Mechanical Properties of Superduplex Stainless Steel Welded Joints

ABSTRACT The tests results of superduplex stainless steel welded joints made with a different heat input, using automatic submerged arc welding (SAW) and semi-automatic flux-cored arc welding (FCAW) have been presented. Metallographic examinations, the measurements of the ferrite content, the width of the heat affected zone (HAZ) and the hardness of the welds in characteristic areas have been performed. Significant differences in the amount of ferrite in the weld metal and in the heat affected zone microstructure of joints were found.

Microstructure Evolution of Ti-6Al-4V Laser Weld Joint HAZ in Superplastic Deformation Process

Microstructure of Ti-6Al-4V laser weld joint heat affected zone was observed. The results suggested that the heat affected zone microstructure first transformed to lamellar structure, then the grain growth and transformed to equiaxial grains. At the same temperature, the grain growth rate was faster at lower deformation rate, and the microstructure was more uniform.

Effect of Preweld Heat Treatment on the Microstructure of Heat-Affected Zone (HAZ) and Weldability of Inconel 939 Superalloy

The effect of two preweld heat treatments on the final microstructure of a heat-affected zone (HAZ) and its effect on the weldability of the superalloy Inconel 939 have been analyzed. The HAZ cracking related to the mechanical driving force criteria during the cooling cycle was directly influenced by the strain state at this zone. Heat treatments result in the formation of (1) irregular γ′ precipitates (more negative misfits) which showed higher strain (high density of interfacial dislocations) and (2) spherical precipitates (near-zero misfits) which as a result of their structure improve weldability by reducing weld HAZ cracking considerably.

The Properties of Low Yield Ratio and High Heat Input Welding Construction Steel and the Microstructure of HAZ

Steel with low yield ratio (YR0.75) and high heat input welding was made by combining Zr, Ti microalloying. Its microstructure investigated by optical and electron microscopes (SEM, TEM), consists of ferrite with the fine pearlite which not only ensures the high strength and toughness but also decreases the yield ratio. The steel was simulated in 100 kJ/cm heat input welding. The microstructures for heat affected zone (HAZ) of weldments mainly consisted of acicular ferrites which nucleate on fine Zr and Ti contained inclusions. The acicular ferrites decrease detrimental influence of grain coursing in the HAZ and make the steel good toughness in low temperature.

Microstructure and Softening of Laser-Welded 960 MPa Grade High Strength Steel Joints

The microstructural evolution of laser-welded 960 MPa grade high strength steel joints and its effect on softening behavior of heat affected zone (HAZ) were investigated in this paper. The results show that microstructure of HAZ and fusion zone (FZ) is composed of lath martensite and bainitic ferrite. The microstructure of mixed grained zone presents strip-like characteristics and small block martensite distributes along the grain boundary. The grain size near the fusion line is about 25 lm, and the grain size in the fine grain zone is less than 5 lm. Microhardness of HAZ and FZ is lower than base metal. The soft zone locates in transitional region between tempering zone and mixed grained zone due to the interaction of the martensite tempering and the recovery and recrystallization of the rolled microstructure. Microhardness of soft zone is 310 HV and drops 18% compared to base material. Welding heat input has a remarkable effect on the width of soft zone and microhardness. The tensile properties of weld joints are closely related to the softening behavior of HAZ.