Study On Microstructures And Properties Of Welded Joints Of ASTM 4130 Steel

ASTM 4130 steel is applied to high pressures pipelines of offshore production platforms because of its excellent mechanical properties. These high pressures pipelines have high carbon contents, high strength levels and thick wall, besides, as a king of quenched and tempered medium carbon steel, so welding of ASTM 4130 steel for piping applications offshore can be difficult, susceptible to problems such as weld defects, hydrogen cracking and hardening and softening of heat-affected zone (HAZ), and then welded joints may be the area with weak performance. This thesis takes ASTM 4130 steel as the research object, studying the features of microstructure and properties of base metal and welded joints by weldability tests. X-ray inspection, optical microscopy (OM), transmission electron microscope (TEM) and scanning electron microscopy (SEM) were imployed to characterize the specimens, and hardness, tensile and low-temperature impact tests were conducted to evaluate the properties of welded joints.The microstructure of ASTM 4130 steel is mainly tempered sorbite. The carbon equivalent value reaches 0.73 to 0.80, and the weldability is poor, so ASTM 4130 steel must be preheated before welding. To achieve favorable comprehensive mechanical properties of welded joints, multi-layer and multi-pass welding and temper bead should be adopted. At the same time, the following procedures were strictly controlled, including preheating at 250±10℃, postweld heat treatment (PWHT) process at 645±10℃for 135～165min, heating and cooling rate was less 150℃/h.In the implant test, the critical crack stress values of preheating at 100℃and 200℃are 642.55 MPa and 806.11 MPa, increasing 86.5% and 134.0% than that at room temperature, respectively. These show that the values gradually increase with preheating temperature. In the Max. hardness test of welding HAZ, Max. hardness value of HAZ presents to the vicinity of weld interface, which achieves at room temperature, dued to the formation of brittle martensite, reaching 566 HV₁₀. The peak values reduce to 455～490 HV₁₀ after preheating at 150℃, 200℃and 250℃, which drop to 345 HV₁₀ at 300℃. Testing proved that preheating can effectively decrease the content of hardening structure, relieving the sensitivity of ASTM 4130 steel. The coarse-grain HAZs, which are in the implant test and Max. hardness test of welding HAZ, consists predominantly of martensite and bainite.The microstructures of weld metal as welded are acicular ferrite and a small amount of pearlite, and which of HAZ are martensite, ferrite and bainite. After PWHT, the microstructure of weld metal consists predominantly of ferrite, a small amount of pearlite and granular carbide, which of HAZ are comprised of tempered sorbite, ferrite and bainite. The carbide gradually precipitates and the structure becomes fine as prolonging the soaking time.With prolonging the soaking time, the tensile strength values increase first and then decrease, and the hardness levels of weld metal have a little change, while which of HAZ and base material gradually reduce. Maximum hardness value presents to the vicinity of weld interface. The hardness values reduce gradually as increasing the distance from weld interface, reaching minimum value nearby 3～4mm from weld interface, and there is an obvious softening phenomenon in HAZ.The impact test results show that weld and fusion zone are the weakest parts of welded joints, and the values of fusion line tend to fluctuate wildly. Base materials have the highest toughness. There is thus a good improvement in the HAZ toughness in spite of minor changes in weld and fusion line as prolonging the soaking time, related to the reduction of coarsened phases and upper bainite in the microstructure.