Laboratory development of high performance steel plates with excellent HAZ toughness at large heat inputs

Presented in this study is the result of steel plates developed at laboratory by using the technique of chemistry design based on microstructure evolution.It has been shown that the produced 50mm thickness steel plates with yield and tensile strength being 420 MPa and 530 MPa respectively exhibit excellent large heat input weldability:the Charpy impact tests in the whole range of heat affected zone(HAZ) including the fusion line at the welded joint with large heat input of 100 -300 kJ/cm showed uniform impact toughness of above 140 J at -40℃.Welding simulations were also performed for heat inputs of 200-600 kJ/cm,which showed far better toughness at -20℃.Analysis on the results of the simulations and the practical welding tests were done and the microstructure evolution mechanisms were proposed.Finally suggestions were given to improve the simulation processes as well as chemistry modification.

Effects of welding wire composition and welding process on the weld metal toughness of submerged arc welded pipeline steel

The effects of alloying elements in welding wires and submerged arc welding process on the microstructures and low-temperature impact toughness of weld metals have been investigated. The results indicate that the optimal contents of alloying elements in welding wires can improve the low-temperature impact toughness of weld metals because the proeutectoid ferrite and bainite formations can be suppressed, and the fraction of acicular ferrite increases. However, the contents of alloying elements need to vary along with the welding heat input. With the increase in welding heat input, the contents of alloying elements in welding wires need to be increased accordingly. The microstructures mainly consisting of acicular ferrite can be obtained in weld metals after four-wire submerged arc welding using the wires with a low carbon content and appropriate contents of Mn, Mo, Ti-B, Cu, Ni, and RE, resulting in the high low-temperature impact toughness of weld metals.

Effect of the heat input on microstructure and properties of submerged arc welded joint of 08Cr19MnNi3Cu2N stainless steel

SAW308L submerged arc welding wire and SJ601A submerged arc welding flux were selected to weld the 12 mm 08Cr19MnNi3Cu2N low nickel and high nitrogen austenitic stainless steel plates with three different welding heat input,and microstructure,tensile properties,microhardness and corrosion properties of the welded joints were studied.The results show that no defects are found in the three groups of welded joints,and the welded joints have better performance.The tensile strength of 08Cr19MnNi3Cu2N stainless steel welded joints with different heat input is slightly lower than that of the base metal,and fracture occurs in the weld zone,and the hardness of the weld zone is lower than that of the base metal.The weld microstructure of stainless steel welded joints with different heat input is composed of austenite+δferrite,and ferrite is uniformly distributed in austenite.With the increase of the welding heat input,the ferrite content in the weld zone decrease gradually,the grain size in the thermal affected zone increase gradually,and the impact toughness reduce.

Effect of welding heat input on HAZ character in ultra-fine grain steel welding

In this essay, we studied how heat input affected the microstructure, hardness, grain size and heat-affected zone(HAZ) dimension of WCX355 ultra-fine grain steel which was welded respectively by the ultra narrow-gap welding (UNGW) process and the overlaying process with CO 2 as protective atmosphere and laser welding process. The experimental results show when the heat input changed from 1.65 kJ/cm to 5.93 kJ/cm, the width of its HAZ ranged from 0.6 mm to 2.1 mm.The average grain size grew up from 2～5 μm of base metal to 20～70 μm and found no obvious soften phenomenon in overheated zone. The width of normalized zone was generally wide as 2/3 as that of the whole HAZ, and the grain size in this zone is smaller than that in base metal. Under the circumstance of equal heat input, the HAZ width of UNGW is narrower than that of the laser welding.

Progress in the technological development of oxide metallurgy for manufacturing steel plates with excellent HAZ toughness

In the present study, the market needs for the （HAZ） toughness are analyzed, and the mechanism of the development of steel plates with excellent heat affected zone decrease in the HAZ toughness during high-heat input welding is discussed.The important countermeasure for improving the HAZ toughness is to employ the technology of oxide metallurgy ,namely,to make use of fine inclusion particles for improving the microstructure of HAZ. The progress and theories of oxide metallurgy technologies developed in the Nippon Steel Corporation （ NSC）, the JFE Steel Corporation and the Kobe Steel Group are illustrated. Steel plates developed by these three companies with excellent HAZ toughness are introduced.

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.

Laser Beam Welding of 600 MPa Quenched and Tempered High-Strength Steel

Conventional fusion arc welding of high-strength quenched and tempered steel can be improved through the use of non-conventional laser beam welding. This article presents the investigations of autogenous bead on plate and butt CO2 Laser Welding (LW) of 7 mm thick high-strength quenched and tempered low alloy SM570 (JIS) steel plates. The influence of laser welding parameters, mainly welding speed, defocusing distance and shielding gas flow rate on the weld profile, i.e., weld zone penetration depth and width, microstructure and mechanical properties of welded joints was determined. All welded joints showed smooth and uniform weld beads free from superficial porosity and undercuts. The selected best welding conditions were a laser power of 5.0 kW, welding speed of 500 mm/min, argon gas shielding flow rate of 30 L/min and a defocusing distance of -0.5 mm. It was observed that these conditions gave complete penetration and minimized the width of the weld bead. The microstructure of the welded joints was evaluated by light optical microscopy. The weld metal (WM) and heat-affected zone (HAZ) near weld metal achieved maximum hardness (355 HV). The tensile fractured samples showed the ductile mode of failure and ultimate tensile strength of 580 MPa.

Metallurgical characteristics of armour steel welded joints used for combat vehicle construction

Austenitic stainless steel(ASS) and High nickel steel(HNS) welding consumables are being used for welding Q&T steels, as they have higher solubility for hydrogen in austenitic phase, to avoid hydrogen induced cracking(HIC) but they are very expensive. In recent years, the developments of low hydrogen ferritic steel(LHF) consumables that contain no hygroscopic compounds are utilized for welding Q&T steels. Heat affected zone(HAZ) softening is another critical issue during welding of armour grade Q&T steels and it depends on the welding process employed and the weld thermal cycle. In this investigation an attempt has been made to study the influence of welding consumables and welding processes on metallurgical characteristics of armour grade Q&T steel joints by various metallurgical characterization procedures. Shielded metal arc welding(SMAW) and flux cored arc welding(FCAW) processes were used for making welds using ASS, LHF and HNS welding consumables. The joints fabricated by using LHF consumables offered lower degree of HAZ softening and there is no evidence of HIC in the joints fabricated using LHF consumables.

The Influence of Welding Parameters on Brittle Fracture of Liquefied Natural Gas Storage Tank Welded Joint

Many applications operate at sufficiently low temperature conditions where most structural steels become very brittle and, therefore, unsuitable for use in safety-critical structures. So the materials used in the vessels or storage tanks which keep the natural gas at liquefaction temperatures need to remain ductile and crack resistant with a high level of safety. The material also needs to have high strength in order to reduce the wall thickness of the container and it must permit welding without any risk of brittle fracture. 9% Ni steel plates are one of most common used materials in the LNG storage tank application. However, the welding procedure for 9% Ni steel plates requires high level of skills of welding that is strictly controlled welding parameter for balancing avoidance of cold and hot cracking and maintenance of high strength. Mechanical properties are important characteristics of the weldment that must confirm to the application feasibility as well as functional requirements of the welded joint. The only way to enhanced the mechanical properties of welded joint by controlling the parameters of using welding process. From the main variables of the arc welding process are the heat input and interpass temperature where the two variables control the thermal cycle of welding process. The experiment show that for thin test specimen with thickness ≤ 14 mm, the heat input range from 1.4 to 2 KJ/mm and controlling interpass temperature within 80°C give high tensile strength with improving the toughness properties of welded joint and reduce the probability of brittle fracture happened by increase the ductility and reduce the yield strength and increased the transition temperature.

Effect of Thermal Cycle on Microstructure and Fracture Morphology in HAZ of HQ130 Steel

The effect of different peak temperature(Tp) and cooling time (t8/5) on microstructure, hardness, impact toughness and fracture morphology in the heat-affected zone (HAZ) of HQ130 steel was studied by using weld thermo-simulation test. Experimental results indicate that the impact toughness and hardness decrease with the decrease of Tpor increase of t8/5 under the condition of a single thermal cycle. There is a brittle zone in the vicinity of Tp= 800℃, where the impact toughness is considerably low. There is a softened zone in the vicinity of Tp=700℃, where the hardness decreases but the toughness increases. In the practical application of multi-layer and multipass welding, the welding heat input should be strictly limited (t8/5≤20s) so as to reduce the softness and brittleness in the HAZ of-HQ130 steel.

EFFECTS OF THERMAL CYCLE ON MECHANICAL PROPERTIES AND FRACTOGRAPHY IN HAZ OF HQ130 STEEL

The effect of different peak temperature T_P) and cooling time (t_(8/5)) on hardness,impact toughness and fracture morphology in the heat--affected zone (HAZ) of HQ130steel was studied by using welding thermo--simulation test. Experimental results showthat the impact toughness and hardness decrease with the decrease of T_P or increase oft_(8/5) under the condition of a single thermal cycle. There is a brittle zone in the vicinityof T_P=800℃, where the impact toughness is considerebly low. There is softened zonein vicinity of T_P=700℃, Where the harkness decreases but the toughness increases. Inthe practical application of multi--layer and multi--pass welding, the welding heat inputshould be strictly limited (t_(8/5)≤20s) so as to reduce the softness and brittleness in theHAZ of HQ130 steel.

On the heterogeneous microstructure development in the welded joint of 12MnNiVR pressure vessel steel subjected to high heat input electrogas welding

Microstructure features of 12 MnNiVR pressure vessel steel welded joint deposited by the high heat input electrogas welding have been systematically investigated. It is revealed that the welded joint is featured by a heterogeneous juxtaposition. The coarse grained heat-affected zone(CGHAZ) primarily consists of lath bainites and minor granular bainites. The fine grained heat-affected zone(FGHAZ) is dominated by polygonal ferrites, pearlites, and fine cementite particles. Moreover, electron backscatter diffraction results further demonstrate that the CGHAZ is populated by coarse prior austenite grains(PAGs) with high frequency(61.3%) of low angle grain boundaries(LAGBs). On the other hand, the FGHAZ is filled with fine PAGs with a lower frequency(19.6%) of LAGBs. Such microstructural differences may likely contribute to differed mechanical properties for samples tested at designed positions.

Microstructure Characterization of High-heat-input Welding Joint of HSLA Steel Plate for Oil Storage Construction

In this paper, microstructure and mechanical properties of welding metals in 610 MPa high strength low alloy （HSLA） were studied after high-heat-input welding. Both the base material and the weld joint proved excellent strength and toughness by vibratory electrogas arc （VEGA） welding under 90 to 100 kJ/cm heatinput. The heat-affected zone （HAZ） was comprised of fine-grain zone （FGZ） and coarse-grain zone （CGZ）, which characterizes fine granular structure and lathing-bainite substructure, It has found that large quantity of dispersed TiN and M23E6 precipitates restrain structure growing in HAZ and strengthen the weldment together with dislocations in the welded joint.

Low Cost High Performance Steel Plate With Superior HAZ Toughness for Large Heat Input Welding

In this paper, a laboratory study has been made to develop low cost high performance steel plates with superior HAZ toughness for large heat input welding. Simulated results show that the absorbed impact energy of heat-affected zone (HAZ) at -20℃reaches above 200J when large heat inputs of 100 to 400kJ/cm were applied, suggestive of superior HAZ toughness for large heat input welding of developed steel plate. The microstructures in HAZ are transformed from mainly fine ferrite and bainite at 100kJ/cm, through an intermediate stage of ferrite, bainite and pearlite at 200 and 300kJ/cm, to nearly fine ferrite and pearlite at 400kJ/cm. The prior austenite grain size and ferrite grain size in HAZ are controlled to ～50 and ～20μm, respectively. The high HAZ toughness is due to the inhibition of prior austenite grain size at high temperatures and the formation of beneficial microstructures to HAZ toughness during continuous cooling.

Influence of Ti on Weld Microstructure and Mechanical Properties in Large Heat Input Welding of High Strength Low Alloy Steels

The influence of Ti on weld microstructure and mechanical properties in large heat input welding of high strength low alloy steels is investigated. The results indicate that a moderate amount of Ti is still effective for grain refinement even under larger heat input and a large amount of acicular ferrite （AF） is formed in the weld metal when Ti content is within 0. 028%--0. 038%. With increasing Ti content, proeutectoid ferrite in the weld metal decreases, whereas bainite and M-A constituent increase. The type of inclusion in the welds varies from Mn-Si-AI-O to Ti-Mn- A1-O and finally to Ti-A1-O as Ti content increases from 0 up to 0. 064%. As for adding 0. 028%--0. 038% Ti, high weld toughness could be attained since most inclusions less than 2 tim which contain Ti20s provide the effective nu- clei for aeicular ferrite formation. However, the toughness of the weld metals severely reduces when Ti content is over the optimum ranRe of 0. 028%--0. 038%.

IMPROVEMENT OF TYPE IV CRACKING RESISTANCE OF 9Cr HEAT RESISTING STEEL WELDMENT BY BORON ADDITION

Creep lives of high Cr ferritic heat resisting steel weldments decrease due to Type Ⅳ fracture, which occurs as a result of formation and growth of creep voids and cracks on grain boundaries in fine-grained heat affected zone (HAZ). Because boron is considered to suppress the coarsening of grain boundary precipitates and growth of creep voids, we have investigated the effect of boron addition on the creep properties of 9Cr steel weldments. Four kinds of 9Cr3WSCoVNb steels with boron content varying from 4.7×10-5 to 1.8×10-4 and with nitrogen as low as 2.0×10-5 were prepared. The steel plates were welded by gas tungsten arc welding and crept at 923K. It was found that the microstructures of HAZ were quite different from those of conventional high Cr steels such as P91 and P92, namely the fine-grained HAZ did not exist in the present steel weldments. Boron addition also has the effect to suppress coarsening of grain boundary carbides in HAZ during creep. As a result of these phenomena, the welded joints of present steels showed no Type Ⅳ fractures and much better creep lives than those of conventional steels.

Heterogeneous Microstructure-Induced Mechanical Responses in Various Sub-Zones of EH420 Shipbuilding Steel Welded Joint Under High Heat Input Electro-Gas Welding

Heterogeneous microstructure-induced mechanical responses in EH420 shipbuilding steel welded joint by electro-gas welding processed have been systematically studied by scanning electron microscopy,electron backscatter diffraction and mechanical testing.Comparing with the coarse-grained heat-affected zone(CGHAZ),the weld metal presents higher toughness(129.3 J vs.37.3 J)as it contains a large number of acicular ferrites with high-angle grain boundaries(frequency 79.2%)and special grain boundary∑3(frequency 55.3%).Moreover,coarse austenite grains in CGHAZ and slender martensite–austenite constituents between bainite laths may likely facilitate crack propagation.Polygonal ferrites and tempered pearlites formed at the junction of the fine-grained heat-affected zone and the intercritical heat-affected zone induced a softened zone with an average hardness of 185 HV0.5,which is the main reason for the occurrence of tensile fracture.

Development of oxide metallurgy technology at Baosteel

Oxide metallurgy technology has been developed at Baosteel for improving the heat affected zone toughness of steel plates for high heat input welding. After deoxidation with strong deoxidizers of Mg alloy, the complex inclusions containing parts of compounds of MgO,Ti2 O3,MnO, Al2 O3 ,MnS, CaS and TiN are formed. These kinds of inclusions are beneficial for promoting the formation of intragranular ferrite. After two-electrode vibratory electrogas arc welding with the V-type groove and a high heat input of 395 kJ/cm in a single pass,the former austenite grain is very fine in size,with an average grain size of only 85 μm. Excellent heat affected zone toughness is obtained for the developed steel plates with a thickness up to 68 mm.

Review of GTAW Welding Parameters

Welding as a fabrication process is one of the vital production routes for most manufacturing industries. Several factors are involved in the choice of welding process for specific applications;notable among these are compositional range of the material to be welded, the thickness of the base materials and type of current. Most metals oxidize rapidly in their molten state, and therefore, the weld area needs to be protected from atmospheric contamination;this is achieved in gas tungsten arc welding GTAW by a shielding gas (argon, helium, nitrogen). GTAW technique is one of the major processes for joining austenitic stainless steels (ASS) and ferritic stainless steel (FSS) fabrication. However, the microstructural change that occurs during welding and at weld joint is still a major challenge today as it affects both the corrosion resistance and the mechanical properties. Therefore, this present paper reviews past research findings on GTA welding of ASS and FSS. Results of the findings have confirmed that, depending on the amount of heat input, which can be controlled by welding parameters (welding speed, voltage and current), welded joints particularly, heat affected zones (HAZs) of both grades of steels can undergo mechanical failure and can be susceptible to corrosion attack if the joints are produced with a less ideal combination of welding parameters.

Microstructure and properties of SAF2507 superduplex stainless steel welded joints

SAF2507 plates （ 12 mm thickness ） were welded using shield metal arc welding （SMAW） process with E2594 electrode. The microstructure, o-phase, and impact fraetograph of the welded joints were analyzed using optical microscope and scanning electron mieroseope. The results show that the fusion zone consists of ferrite, chromium nitride, and secondary aastenite precipitation when welding is performed at low heat input （0. 5 kJ/mm）. However, the increase in heat input causes precipitation of brittle o＂ phase at the y/c~ interface in weld metal and heat-affected zone, as well as a brittle fracture along the grain boundary. Heat input in the range of O. 5 kJ/mm to 1.5 kJ/mm is suitable for joining SAF2507 plates.