Hardness In HAZ Research Articles

Problems with reliability and safety of hot dip galvanized steel structures

Hot dip galvanizing is very effective means of protection against corrosion. Some recommendation concerning the steel quality are generally known and accepted. The process consists of cleaning (pickling or sand blasting) and dipping the structures or pieces into liquid zinc bath. The case study of hot dip galvanized steels is presented. Some recent failures of hot dip galvanized welded structures and hot dip galvanized high strength steel screws are presented. Structures were made of S355 grade steel and MIG/MAG process was applied for welding. Large cracks were observed in the vicinity of welds after hot dip galvanizing process. The presence of both hydrogen and liquid metal embrittlement was identified and associated mainly with higher hardness of HAZ or the quenched and tempered steels. Possible cracking mechanisms are discussed. The influence of chemical composition and production process (welding, heat treatment) was analyzed according to data published in literature. The solutions and recommendations for avoiding the failure in hot dip galvanized structures are proposed.

Impact of atmosphere pressure on all-position weld shaping of hyperbaric welding

The influence rule of air environment pressure on all position primary welding parameters and welding quality under hyperbaric environment is investigated. The investigation shows that the environment pressure has some effect on weld penetration. Primary welding current of downhand welding decreases linearly with the ambient pressure increasing from 0.1 MPa to 0.7 MPa; Primary welding current of overhead position welding and upward welding in the inclined position decreases greatly with the ambient pressure increasing from 0.1 MPa to 0.3 MPa and then changes little afterwards. Primary welding current of downward welding in the incline presents lenitive descent trend with the ambient pressure increasing from 0.1 MPa to 0.7 MPa. Ambient pressure has great affection on mechanical property of welding joint, tensile strength and impact toughness of joint decrease with the ambient pressure increasing, while the hardness of HAZ and welding seam increase.

Control of Hardness Distribution in Friction Stir Welded AA2024-T3 Aluminum Alloy

AA 2024-T3 Aluminum alloy plates of 3 mm thickness were friction stir butt welded at a constant rotation speed of 1250 min � 1 and welding speeds of 50 and 100 mm/min. Three types of backing materials such as SUS304, pure copper block, and a combination of copper-block with 0.5 mm SUS304 were used. Controls of temperature history were achieved and their effects on microstructures, hardness distributions, and tensile properties of the joints were investigated. Grain size of the stir zone decreased from 7.4 mm in the case of stainless steel to 4.4 and 1.7 mm for the cases of a combination backing type and copper-block respectively. As a result of higher heating/or cooling rates, both higher welding speed and copper-block backing material resulted in increased hardness in HAZ and a maximum value was achieved for the combined backing plate at 100 mm/min of welding speed. Higher peak temperature was beneficial for higher hardness in stir zone while lower one in HAZ. A maximum tensile strength of the joint was achieved at a welding speed of 100 mm/min when the combined backing type was used. The joint efficiency of this case was 93.6% and this value was the highest among the present joints. [doi:10.2320/matertrans.47.1560]

Microstructure and Mechanical Properties of Hyper-Interfacial Bonded Joints of Ultra-Fine Grained High Strength Steels

A new conceptual bonding technique, " hyper-interfacial bonding" has been proposed as the most effective bonding technique for ultra-fine grained steels. The hyper-interfacial bonding process was characterized by the instantaneously surface-melted pressure-bonding which involved a series of steps, namely, surface heating by high frequency induction, rapid removing of a heating coil and simultaneously pressing of specimens together by an oil pressure system. All equences were typically completed within a second under vacuum/gas atmosphere. An ultra-fine grained high strength steels with the average grain size of 1.0-1.5µm were used for bonding. A bainitic structure and MAconstituents were confirmed in the HAZ, while the base metal indicated the fine ferrite-cementite/ pearlite texture. The maximum hardness in HAZ was limited at HV320-400 and the softening in the HAZ did not occur consequently. Prior austenitic grains were coarsened at the vicinity of the bond interface, however, the grain growth in the bond layer could be depressed below about 11-16µm attributed to the dynamic recrystallization during pressure-bonding process. The tensile strength of joints attained to 83% of the base metal strength. The upper-shelf absorbed energy and DBTT were improved to approx. 60J/cm2 and 223K respectively in the case of Ni-plated UFG steel joints. It could be resulted that the hyper-interfacial bonding technique was a feasible joining method for ultra finegrained steels.

Cold Cracking in Weldments of Steel S 690 QT

The cold cracking process in shielded metal arc and gas metal arc welding of steel S 690 QT is investigated by Tekken test. The conditions for cold cracking are varied by changing the specimens’ thickness, heat input, initial weld metal hydrogen concentration and preheating temperature. The kinetics of initiation and propagation of cracking is monitored by acoustic emission equipment. It is found out that the crack’s propagation path shifts from weld metal to fusion line and HAZ with decreasing initial weld metal hydrogen concentration and increasing hardness of HAZ. In the investigated range of cold cracking conditions the magnitude of cracking is mainly controlled by the preheating temperature and heat input. The acoustic emission signal provides valuable information about the factors controlling the kinetics of crack initiation and propagation and the intensity of cold cracking. The initial hydrogen concentration in weld metal controls the incubation period’s duration and affects the cracking intensity at higher heat inputs. The preheating temperature does not influence the incubation period, but significantly affects the kinetics of crack propagation. The heat input has a complex influence on the cracking kinetics and intensity. This is related to the contradictive effects of heat input on the initial hydrogen concentration per unit length of weld metal and on the behaviour of hydrogen during cooling. The obtained results provide a basis for combined experimental — modelling investigations on the cold cracking phenomenon in weldments of higher strength structural steels, aiming at quantitative evaluation of the influence of the main controlling factors.

Effects of welding heat input on microstructure and hardness inheat-affected zone of HQ130 steel

There are two parts of the heat input in gas-metal arc welding (GMAW): one isthe arc heat flux and the other is the heat content of filler metal droplets.This paper introduces a new mode of arc heat flux density distribution on thedeformed GMAW weldpool surface. A numerical model of fluid flow and temperaturefield in GMAW is established according to the new mode of arc heat fluxdistribution. By using a numerical simulation technique, the effects of weldingheat input on microstructure and hardness in HAZ of HQ130 steel are studied. Thedimensions of Austenitic grains and hardness in different locations in HAZ ofHQ130 steel are calculated under different welding heat inputs. Experiments showthat the calculated results of welding thermal cycle and microstructure andhardness in HAZ of HQ130 steel are in agreement with measured ones.

Creep rupture strength of heat affected zone for 9Cr ferritic heat resisting steels

This paper describes creep rupture characteristics of weld heat affected zone, HAZ for 9Cr ferritic steels that are promising materials for nuclear energy uses. In general, creep rupture strength in the heat affected zone of peak temperature between 900 and 1000°C is lower than that in the base metal for ferritic steels. Grain refinement and coagulation of carbides for 9Cr–1Mo steels cause decrease in creep rupture strength of the HAZ. The hardness in the simulated HAZ heated to around 1000°C decreases during creep. This seems to be related to weakening of the HAZ at 1000°C. However, substitution of W for Mo is very effective in enhancement of creep rupture strength of the HAZ due to higher stability of carbides and increase in quantity of precipitated carbides during creep rupture test.

Characteristics of Pulsed Current Bead on Plate Deposit in Flux Cored GMAW Process.

The influences of pulse parameters, arc voltage and welding speed on the characteristics of weld beads deposited on C-Mn steel plate by flux cored GMAW process have been studied. The pulse parameters are found to affect the microstructure and porosity content of weld bead and the width of HAZ. The arc voltage, affecting the lp, and lb, also found to influence the characteristics of weld bead significantly. The porosity content of weld bead and the width and hardness of HAZ are found to have a significant correlationship with a factor, defined as a function of lp, lb, f and tb. The influence of welding speed has been found of usual nature. It is marked that a right selection of pulse parameters can improve the weld quality over that observed in case of continuous current GMAW.

中・高炭素低合金鋼の溶接熱影響部における低温割れの研究 Ｖ 斜めＹ型溶接割れ試験法による焼割れ的低温割れ発生の防止条件の検討

The effect of preheating on quenching crack type cold cracking in HAZ of JIS SNCM439 and SK5 has been studied by the y-slit cracking test utilizing GTA welding with duplex stainless filler wire without the effect of diffusible hydrogen. Furthermore, the validity of the crack free condition which was proposed in the previous paper by the simulated cold cracking test under the various materials and cooling conditions has been also studied by use of the results of the y-slit cracking test.In SNCM439, it was noted that cracking do not occur at somewhat low preheating temperature, although the hardness and microstructure (martensite with some retained austenite) are nearly the same as that of without preheating. Therefore, it was found that cracking in this steel can be avoidable by adoption of lower preheating even in the range of cooling rate without bainite. In SK5, cracking has a tendency to decrease gradually with decreasing in hardness through the increase in formation of troostite and bainite by increasing preheating temperature.According to the crack free condition of the hardness less than about Hv600 or the fracture stress more than about 950 MPa suggested in the previous paper, it was confirmed that cracking do not occur under the condition of the restraint intensity 15 kN/mm⋅mm in the y-slit cracking test. Consequently by use of the relationship between the hardness and the fracture stress, the critical hardness in HAZ for cracking can be easily estimated for various restraint intensities in actual weldedjoint.

Influence of positioning of electrode on the morphology of HAZ in submerged arc welded SA-203 steel.

During multipass submerged arc welding of thick alloy steel plates for special applications such as cryogenic and pressure vessel the consideration of HAZ morphology governing its toughness is of utmost importance. Besides the current voltage and welding speed the positioning and the angle of attack of the electrode are having a considerable influence on the thermal cycle of HAZ. In this work the effects of angle of attack of the electrode to workpiece, θ, (5 to 45°) and the distance between the electrode and the weld groove edge, φ, (0.4 to 3.0mm) on the morphology and the width of the HAZ have been studied during multipass submerged arc welding of a 108mm thick ASME-SA-203 Gr.B-582 steel plate. At a given φ the increase in θ enhances the width of HAZ and the grain size in HAZ adjacent to the fusion line. At a given θ the increase in φ reduces the width of HAZ and the grain size in HAZ adjacent to the fusion line. The hardness of HAZ measured in the region near to the fusion line has been found to increase with the increase in φ under a given θ. However, at a given φ the increase in θ has been found to decrease the hardness of HAZ. The variation in hardness of HAZ has been found to be influenced primarily by the presence of some small second phase colonies having a comparatively high hardness in the matrix. It has also been observed that the standard deviation of hardness in HAZ in general is comparatively lower when the welding is carried out at a relatively low θ (20°) by keeping φ not less than 1.0mm.

Effect of fabrication procedure on liquid metal embrittlement cracking in welded steel structures by molten zinc

This study deals with the estimation of occurrence and preventive condition of the Liquid Metal Embrittlement cracking (LMEC) due to hot dip galvanizing using Y-Groove Crack ing Test pieces.The results obtained are as follows, (1) Using Y-Groove Cracing Test piece is appropriate for the estimation test of the LMEC in welded steel structures.(2) The critical condition for LMEC occurrence is indicated by the relation between the intensity of restraint and the maximum hardness in HAZ (Hv max.).(3) The LMEC preventive condition for welded steel structures to be galvanized are obtained by the Hv max. under the condition that welded joints have the most severerity value of the intensity of restraint in each type of structure. For example, the LMEC preventive condition of weldment in transmission tubular tower structures is assumed that Hv max. keeps less than 260.

溶接鋼構造物の溶融金属脆化割れに及ぼす施工条件の影響

This study deals with the estimation of occurrence and preventive condition of the Liquid Metal Embrittlement cracking (LMEC) due to hot dip galvanizing using Y-Groove Crack ing Test pieces.The results obtained are as follows, (1) Using Y-Groove Cracing Test piece is appropriate for the estimation test of the LMEC in welded steel structures.(2) The critical condition for LMEC occurrence is indicated by the relation between the intensity of restraint and the maximum hardness in HAZ (Hv max.).(3) The LMEC preventive condition for welded steel structures to be galvanized are obtained by the Hv max. under the condition that welded joints have the most severerity value of the intensity of restraint in each type of structure. For example, the LMEC preventive condition of weldment in transmission tubular tower structures is assumed that Hv max. keeps less than 260.

溶融亜鉛めっき時における溶接継手の液体金属ぜい化 Ｉ 溶融亜鉛中における溶接継手強度に及ぼす溶接諸因子の影響

It is now questionable that the Liquid-Metal-Embrittlement-cracking (LME-cracking) rises in HAZ of welds of steel structure during hot dip galvanizing. Because of preventing the cracking, it is necessary to study the effect of welding factors on the critical stress of steel in molten zinc (σcr).In this report, the tensile tests were carried out in molten zinc by using the welding joint specimen with U notch in HAZ, and σcr for each materials used was investigated. The results are as follows.(1) σcr decreased as the stress concentration (Kt) was large, but it was approximately constant over Kt=3.(2) As the heat input was large, grain size at coarse-grained region was large, maximum vickers hardness in HAZ was lower and σcr increased.(3) σcr was linearly arranged by using maximum vickers hardness in HAZ.