MICROSTRUCTURE AND INCLUSION CHARACTERIZATION IN THE SIMULATED COARSE-GRAIN HEAT AFFECTED ZONE WITH LARGE HEAT INPUT OF A Ti-Zr-MICROALLOYED HSLA STEEL

The microstructure and the characteristics of the inclusions embedded in ferrite matrix in simulated coarse-grain heat affected zone (CGHAZ) of a Ti-Zr-treated high strength low alloy (HSLA) steel have been investigated. The microstructure of the simulated CGHAZ dominantly consisted of intragranular acicular ferrite (IAF) combining with a small amount of polygonal ferrite (PF), widmanst tten ferrite (WF), bainite ferrite (BF), pearlite and martensite-austenite (M-A) islands. The PF, WF and BF were generally observed at the prior austenite grain boundaries and the interlocking acicular ferrite was usually found intragranularly. It was found that the inclusions were composed of Ti2O3, ZrO2, Al2O3 locating at the center of the particles and MnS lying on the surface layer of the inclusions. The intragranular complex inclusions promoted the acicular ferrite formation and the refinement of microstructure whilst those at prior austenite grain boundaries caused PF formation on the inclusions. The simulated CGHAZ consisting of such complicated microstructure exhibited desired mechanical properties.

EFFECT OF Zr ADDITION TO Ti-KILLED STEEL ON INCLUSION FORMATION AND MICROSTRUCTURAL EVOLUTION IN WELDING INDUCED COARSE-GRAINED HEAT AFFECTED ZONE

Effects of Zirconium on the chemical component and size distribution of Ti-bearing inclusions, favored the grain refinement of the welding reduced, coarse-grained heat affected zone （CGHAZ） with enhanced impact toughness in Ti-killed steels, which were examined based on experimental observations and thermodynamic calculations. It indicated that the chemical constituents of inclusions gradually varied from the TiO oxide to the Ti-O＋Zr-O compound oxide and a single phase of the ZrO2 oxide, as the Zr content increased from zero to 0.0100%. A trace of Zr （0.0030%-0.0080%, depending on the oxygen content in liquid steel） provided a large amount of nucleating core for Ti oxide because of the larger specific density of ZrO2 oxide, and produced a small size distribution of the inclusions favorable for acicular ferrite transformation with a high nucleation rate in the CGHAZ, and a high volume fraction of acicular ferrite was obtained in the CGHAZ, with enhanced impact toughness. Otherwise, a high content of Zr （-0.0100%） produced a single phase Zr02, which was impotent to nucleate acicular ferrite, and a microstructure composed of ferrite side plate and grain boundary ferrite developed in the CGHAZ. The experimental results were confirmed by thermodynamic calculations.

Effect of welding parameters on the heat-affected zone of AISI409 ferritic stainless steel

One of the main problems during the welding of ferritic stainless steels is severe grain growth within the heat-affected zone （HAZ） In the present study, the microstmctural characteristics of tungsten inert gas （TIG） welded AISI409 ferritic stainless steel were investigated by electron backscattered diffraction （EBSD）, and the effects of welding parameters on the grain size, local misorientation, and low-angle grain boundaries were studied. A 3-D finite element model （FEM） was developed to predict the effects of welding parameters on the holding time of the HAZ above the critical temperature of grain growth. It is found that the base metal is not fully recrystallized. During the welding, complete recrystallization is followed by severe grain growth. A decrease in the number of low-angle grain boundaries is observed within the HAZ. FEM results show that the final state of residual sWains is caused by competition between welding plastic strains and their release by recrystallization. Still, the decisive factor for grain growth is heat input.

Study on the microstructure of 12%chromium low carbon stainless steel in a high temperature heat-affected zone

Coarsening, embrittlement and corrosion sensitization in a high temperature heat-affected zone （HTHAZ） are the major problems when 12% chromium low carbon stainless steel is being welded, which induce the deterioration of the impact toughness at a low temperature and intergranular corrosion resistance property. This study investigates the corresponding microstructures in HTHAZ with different chemical compositions and heat inputs through thermal simulation tests. The results show that the martensite content increases with the descending of ferrite factor （FF） when FF is below 9.0 and heat input influences the microstructure of high FF steel in HTHAZ. Martensite of 12% Cr stainless steel in HTHAZ with only Nb stabilization reticularly distributes at ferrite grain boundaries.

Microstructure and mechanical properties of heat affected zone in multi-pass GMA welded Al-Zn-Mg-Cu alloy

Microstructure and mechanical properties of the heat affected zone(HAZ)in multi-pass gas metal arc(GMA)welded Al Zn Mg Cu alloy plates were investigated,based upon which the mechanical anisotropy and fracture mechanism were analyzed.The microstructure and composition were analyzed by scanning electron microscope(SEM)and energy dispersive spectroscope(EDS).X-ray diffractometer(XRD),transmission electron microscope(TEM)and selective area electron diffraction(SAED)were used to analyze the phase composition.The distribution of microhardness was identified as gradual transition and tensile strength had a tendency to decrease first and then increase.The distribution of nano-sizedη(MgZn2)particles in theα(Al)matrix and Al2MgCu phase determined the tensile performances along the thickness direction and led to the formation of ductile/brittle composite fracture in the HAZ.The continuous distribution of Al2MgCu phase in the strip intergranular precipitates gave birth to premature cracks and the brittle fracture region.The precipitated particles coarsening also led to the deterioration of mechanical properties.

In situ observation of austenite grain growth behavior in the simulated coarse-grained heat-affected zone of Ti-microalloyed steels

The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then grew in different directions through movement of grain boundaries into the ferrite phase. Subsequently, the adjacent austenite grains impinged against each other during the α→γtransformation. After the α→γ transformation, austenite grains coarsened via the coalescence of small grains and via boundary migration between grains. The growth process of austenite grains was a continuous process during heating, isothermal holding, and cooling in simulated thermal cycling. Abundant finely dispersed nanoscale TiN particles in a steel specimen containing 0.012wt% Ti effectively retarded the grain boundary migration, which resulted in refined austenite grains. When the Ti concentration in the steel was increased, the number of TiN particles de- creased and their size coarsened. The big particles were not effective in pinning the austenite grain boundary movement and resulted in coarse austenite grains.

Effect of Heat Input and M-A Constituent on Microstructure Evolution and Mechanical Properties of Heat Affected Zone in Low Carbon Steel

Microstructure evolution and impact toughness of simulated heat affected zone（HAZ） in low carbon steel have been investigated in this study. Thermal simulator was used to simulate microstructure evolution in HAZ with heat input of 10-100 kJ/cm welding thermal cycle. Results indicated that microstructure of HAZ mainly consisted of acicular ferrite（AF） inside grain and high volume fraction of grain boundaries ferrite（GBF） at prior austenite boundaries; the size of GBF and effective grain size increased with increasing heat input. Excellent impact toughness（more than 150 J at-40 ℃） was obtained in HAZ with heat input less than 50 k J/cm. When heat input was 100 kJ/cm, the impact toughness of HAZ decreased to 18 J because of the presence of large M-A constituent with lath-form in HAZ, assisting the micro-crack initiation and decreasing the crack initiation energy seriously. Effect of inclusions on acicular ferrite transformation in HAZ was also discussed.

Microstructure and embrittlement of the fine-grained heat-affected zone of ASTM4130 steel

The mechanical properties and microstructure features of the fine-grained heat-affected zone（FGHAZ） of ASTM4130 steel was investigated by optical microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,and welding thermal simulation test.It is found that serious embrittlement occurs in the FGHAZ with an 81.37% decrease of toughness,compared with that of the base metal.Microstructure analysis reveals that the FGHAZ is mainly composed of acicular,equiaxed ferrite,granular ferrite,martensite,and martensite-austenite（M-A） constituent.The FGHAZ embrittlement is mainly induced by granular ferrite because of carbides located at its boundaries and sub-boundaries.Meanwhile,the existence of martensite and M-A constituent,which distribute in a discontinuous network,is also detrimental to the mechanical properties.

The softening prediction of HSLA steel heat-affected zone based on the grey system

The high-strength low-alloy（ HSLA ） steel heat-affected zone （HAZ）softening was predicted using a grey model. HSLA steel DILLIMAX690E, NK-HITEN61OU2 and BHW35 were taken as examples in the research on ultra-narrow gap automatic welding technology. Test results turned out to be that the errors between the values calculated by the Grey Model （GM） （ 1,1 ） model and their actual value were less than 2%, indicating that the grey prediction method could accurately reflect the actual situation of the high-strength low-alloy steel heat-affected zone softening. This method will play a crucial role in guiding the applications of HSLA steel welded structures in the future.

Microstructure and toughness of coarse grain heat-affected zone for Nb-microalloyed X80 pipeline steel

Based on welding thermal simulation on Nb-microaUoyed XSO pipeline stee! using Gleeble-3500 thermal simulation equipment, microstlttcture and impact toughness in coarse grain heat-affected zone （CGHAZ） under different welding parameters were investigated in this paper. The results show that high heat inputs with low preheats or low heat inputs with high preheats should be applied to achieve high impact toughness. Coarse original austenite grains may lower impact toughness. CGHAZ microstructure is mostly composed of upper bainite, granular bainite and lath bainite. The phase composition of microstructure and the quantity, size, shape of M/A constituents both have effects on impact toughness.

Residual Stresses and Micro-Hardness Testing in Evaluating the Heat Affected Zone’s Width of Ferritic Ductile Iron Arc Welds

Shielded Metal Arc Welding (SMAW) in Ductile Irons (DI) is often required by foundries for practical manufacturing reasons. The mechanical properties of the welded structures are strongly dependent on their HAZ’s width. A model based on the behaviour of the ferritic matrix of high-Si DIs in order to make an approach in measuring their HAZ’s width is developed in this study. A series of thermal treatments on 3.35 and 3.75 wt% Si as-cast DIs and spot SMAWs is applied on these materials. The applied SMAWs are done on non-preheated and preheated samples (150℃ - 300℃). For welding we modify the amperage (100 - 140A). The micro-hardness Vickers changes in the ferrite of the as-cast samples and inside the HAZ of the welded ones can be attributed to the existence of residual stresses (RS) in the ferritic matrix and assist in estimating the HAZ’s width.

High-temperature creep properties of fine grained heat-affected zone in P92 weldment

The simulated fine grained heat-affected zone (FGHAZ) specimens for P92 welded joints were prepared by heat treatment, then the creep tests were carried out at 650 ℃ under the applied stress of 90-120 MPa to investigate high-temperature creep behavior of FGHAZ. The results show that the creep property of FGHAZ is much inferior to that of the base metal, which exhibits the much higher steady creep rate and shorter time to creep fracture. The power law equation can describe the steady creep rate dependence on applied stress, indicating that the stress exponent n of FGHAZ is distinguished between two regions with n=15.1 at high stresses (more than 100 MPa) and n=8.64 at lower stresses. Based on Monkman-Grant equation, the relationship between the secondary creep rate and time to rupture is obtained to evaluate the creep life of FGHAZ with the applied stress above 100 MPa.

Effect of Zr-Ti combined deoxidation on impact toughness of coarse-grained heat-affected zone with high heat input welding

In this study, the effects of Zr-Ti combined deoxidation and AI deoxidation on the impact toughness of coarse- grained heat-affected zone in high-strength low-alloy steels were investigated. More fine oxides were formed in the Zr-Ti-killed steel than in Al-killed steel. It was also found that more acicular ferrite grains were formed in the coarse-grained heat-affected zone in the Zr-Ti-killed steel than in Al-killed steel. The impact toughness of coarse-grained heat-affected zone of Zr-Ti-kiUed steel was higher than that of Al-killed steel. The good impact toughness was attributable to the pinning effect of fine oxides and the formation of acicular ferrite grains on fine oxides.

Heat-affected zone in pulsed laser cutting of titanium alloy

For the machining of bladed ring parts of an aeroengine, experiments on Nd∶YAG pulsed laser cutting of TC1 titanium alloy sheet were carried out to investigate the influences of laser cutting parameters on heat-affected zone (HAZ). The selected laser cut samples were photographed by scanning electron microscopy. The results show that the microstructure in HAZ layers is characterized by metastable acicular martensite grains with a slight increase of hardness by 10%. The thickness of HAZ layers was studied as a function of laser cutting parameters. Medium pulse energy, higher pulse rate, higher cutting speed and argon at higher pressures help to reduce the thickness of HAZ layers.

Simulation research on heat-affected zone of the NiTi alloy plasma arc welded joints

In order to study the compositions and properties of NiTi alloy plasma arc welded joints, different temperatures of the heat-affected zone（HAZ） are simulated. Temperatures are set at 20 ℃, 400 ℃, 500 ℃, 600 ℃, 700 ℃, 800 ℃, 900 ℃ and 1 000 ℃. X-ray diffraction results of the thermal simulated specimens show that NiTi2 phase emerges in the alloy when the temperature is between 500 ℃ and 800 ℃, while NiTi2 and Ni3 Ti phases emerge when the temperature is between 800 ℃ and 1 000 ℃. Tensile strengths of specimens at 600 ℃ and 900 ℃ are only 68. 0% and 61.3% of the strength at the room temperature respectively due to the emergence of NiTi2 and Ni3 Ti phases.

Study on laser welded heat-affected zone in new ultralow carbon bainitic steel

800 MPa grade ultralow carbon bainitic （NULCB） steel is the recently developed new generation steel, which was produced by thermo mechanical controlled processing ＆ relaxation-precipitation controlling transformation （TMCP＆RPC） tech- nique. The microstructure and the mechanical properties of the heat-affected zone （HAZ） in NULCB steel under laser welding conditions were investigated by using a Gleeble-1500 thermal simulator. The experimental results indicate that the simplex microstructure in the HAZ is granular bainite that consists of bainite-ferrite （BF） lath and M-A constituent when the cooling time from 800 to 500℃ （t8/5） is 0.3-30 s, and the M-A constituent consists of twinned martensite and residual austenite. As t8/5 increases, the hardness and tensile strength of HAZ decreases, but they are higher than that of the base metal, indicating the absence of softened zone after laser welding. The impact toughness of HAZ increases at first and then decreases when t8/5 increases. The impact energy of HAZ is much higher than that of the base metal when t8/5 is between 3 and 15 s. It indicates that excellent low temperature toughness can be obtained under appropriate laser welding conditions.

Study on local embrittlement of welding heat-affected zone in XSO pipeline steels

The relationship between the microstructure and toughness of welding heat-affected zone in XSO grade pipeline steels is studied. It is found that the intercritical reheated coarse-grained heat-affected zone （ICCGHAZ） of experimental steels has the lowest toughness values when the secondary peak temperature is at intercritical （ α ＋ γ ） region during multi-pass welding. The local embrittlement is mainly attributed to the morphology, amount and size of M-A constituent. It is also found that the microstructural inhabitanee at ICCGHAZ has a deleterious effect on the toughness. On the basis of above experimental results, it is suggested that the local embrittlement should be prevented by using pre-heating thermal cycle which could eliminate the microstructural inhabitance and using post-heating thermal cycle which could improve the morphology, amount and size of MA constituent.

Influence of the secondary welding thermal cycle on the microstructure and property of coarse grain heat-affected zone in an X100 pipeline steel

The influence of the secondary thermal cycle on the microstructure of coarse grain heat-affected zone in an XIO0 pipeline steel was investigated by means of a thermal simulation technique and microscopic analysis method. The property of coarse grain heat-affected zone was characterized by Charpy V-Notch impact properties testing. The results indicated that the experimental steel exhibited local brittleness of intercritically reheated coarse-grained heat-affected zone when the peak tempera- ture of secondary thermal cycle was in the range of two phases region （ ~ and 3/）. There were two main reasons for the local brittleness. The first was that the microstructures of intercritically reheated coarse-grained heat-affected zone were not fined although partial grain recrystallization occurred. The second was that M-A islands, which had the higher content, larger size and higher hardness, existed in intercritically reheated coarse-grained heat-affected zone.

Effects of Electrode Types on the Microstructure, Tensile and Hardness Properties of 304 L Austenitic Stainless Steel Heat-Affected Zone (HAZ)

Submerged arc welding (SAW) has been well utilised for the production of weld joints in 304 L ASS for various industrial application. However, effective performance of the material in service has been hampered by improper choice of electrode. Therefore, in this study, effects of different types of electrode on the microstructure and tensile property of type 304 L austenitic stainless steel heat-affected zone (HAZ) were studied. Chemical composition of the as-received sample was determined. A number of samples were cut from the as-received sample. Afterwards, two half were joined together with 308 L, 312 L and 316 electrodes at a controlled welding speed, current and voltage of 4.6 mm/s, 160 A and 30 V to produce a constant heat input of 626.09 J/mm. An automatic SAW machine with Model Type: DX3-301, and Frequency: 50 Hz was used. And based on ASTM standard, tensile and hardness samples were prepared from the as-received and HAZs. Tensile and hardness measurements were made. Also, specimens for microscopy studies were prepared from the HAZ and as-received samples. From the results, microstructures of the HAZs revealed varied volume fraction of austenite and ferrite phases and grain sizes, and at austenite and ferrite grain boundaries, chromium carbide formation and precipitation were observed. The weld joint produced with 308 L electrode revealed optimum UTS value and YS value of 475 and 325 respectively. While weld joint produced with 316 L electrode has superior ductility of value 41%. Irrespective of the types of electrode used, the as-received sample revealed superior tensile properties over the weld joints. Also, optimum hardness value of 45.7 HRA was obtained with 308 L. Hardness value of the as-received sample was higher than that of HAZ samples.

MICROSTRUCTURE AND MECHANICAL PROPERTY DEVELOPMENT IN THE SIMULATED HEAT AFFECTED ZONE OF V TREATED HSLA STEELS

The simulated heat affected zone （HAZ） of the high strength low alloy （HSLA） steels containing 0%, 0.047%, 0.097% and 0.151% vanadium, respectively, were studied with Gleeble-2000 thermomechanical simulator to determine the influence of vanadium addition on the mechanical properties of the HAZ. The HAZ simulation involved reheating the samples to 1350℃, and then cooling to ambient temperature at a cooling rate of 5℃/s ranging from 800 to 500℃ （△8/5=60s）. The mechanical properties including tensile strength and -20℃ impact toughness were conducted. The microstructures of the base steel and the simulated HAZs were investigated using optical microscope, scanning electron microscope （ SEM ） and transmission electron microscope （TEM）. Based on the systemutic examination, the present work confirmed that about 0.05% vanadium addition to low carbon low alloy steels resulted in expected balance of strength and toughness of the HAZ. And more than 0.10% levels addition led to detrimental toughness of the HAZ SEM study showed that the simulated 0.097% and 0.151%V HAZs consisted of more coarse ferrite plates with greater and more M-A constituents along austenite grain and ferrite plate bound- aries. The impact fracture surfaces of the simulated 0.097% and 0.151%V HAZs showed typically brittle mode with predominant cleavages. The size of the facet in the fracture surface increased with increasing vanadium level from 0.097% to 0.151%.As a result, the simulated 0.151% V HAZ has the lowest impact toughness of the four specimens.