Prediction of fatigue crack growth rate in welding residual stress field

Superior fatigue crack growth properties in newly developed weld metal

Hot crack prevention in materials production and processing is an essential prerequisite for welded component safety. The causes of hot cracking can ultimately be attributed to the occurrence of metallurgical effects and to structural loads. More than 140 hot cracking test procedures have hitherto been developed for determining the hot cracking resistance. In principle, they are divided in self-restraint and externally loaded hot cracking tests with diverse process variants. Only some of the hot cracking tests are international standardized. Although various factors are known that encourage or prevent hot cracking, it is often not possible even with defined welding conditions to draw immediate conclusions about the hot cracking resistance of a welded component alone from a metallurgical composition of the base and filler materials. Based on an evaluation of the existing theories relating to hot cracking susceptibility assessment, this study summarizes the major hot cracking test procedures and highlights the application limits of the test procedures by presenting overviews along with explanations. It shows that weld hot cracking tests can generally be used to rank materials, welding consumables, and welding conditions. The evaluation of hot cracking test results and of their transferability among one another and to real components always requires consideration of the close relationships between metallurgy, welding process, and parameters, respectively, and prevailing restraint conditions.

Cold cracking tests—an overview of present technologies and applications

Fatigue crack propagation rates and threshold stress intensity factors for welded joints of ht80 steel at several stress ratios

The fatigue strength of lap joints of thin steel plate was improved by using a developed low-transformation-temperature welding wire that induced the compressive residual stress around a welded part in the as-welded condition. The transformation of weld metal from austenite to martensite began around 180°C and finished around room temperature. This expansion of weld metal is constrained by the surrounding base metal and induces compressive residual stress. The mean stress effect due to the compressive residual stress improved the fatigue limit approximately 1.5 times.

Fatigue strength improvement of lap joints of thin steel plate using low-transformation-temperature welding wire

The fatigue crack propagation rate and the stress intensity threshold level were measured on welded joints which were made by three kinds of welding processes, manual arc welding with covered electrode, gas metal arc welding, and submarged arc welding The specimens were 200 mm wide center notched type The material was a SM50B steel 20 mm in thickness The fatigue crack propagation properties of the weld metal and the heat affected zone of these welded joints were found to be quite similar to each other and inferior to that of the base metal The stress intensity threshold level of welded joints was about one fourth of that of the base metal It is assumed that the inferiority of welded joints in crack propagation properties may be caused from the tensile residual stresses distributed in the middle part of the specimens The removal of the tensile residual stresses by postweld heat treatment could improve welded joints in the fatigue crack propagation properties The fatigue crack propagation properties and the stress intensity threshold level of welded joints after the postweld heat treatment are similar to those of the base metal

Effect of Residual Tensile Stresses on Threshold Level for Fatigue Crack Propagation in Welded Joints of SM50B Steel

Cooling time, which has an immediate connection with root cracking in a high strength steel weld with its strength level of 60 kg/mm2 and below, from 800 to 300°C was measured for various welding conditions in the cases of welding with coated electrodes and submerged are welding, respectively. Experimental formulas for the estimation of the cooling time were obtained and these nomographs were shown.The main conclusions are summarized as follows:(1) Dimensions of heat flow in a weld were varied by any particular combination of welding condition, plate thickness and the shape of penetration.For the cooling process with two-or three-dimensional heat conduction in a weld, a relation between temperature T on a weld and cooling rate (dT/dS) as the temperature T is shown as follows :(dT/ds)T∞(T-T0)nwhere T0: initial temperature of platen: the exponent which is varied with dimensions of heat flow in a weldHence, the dimensions of heat flow was clearly shown as the gradient of a line in a logarithmic groph of (dT/dS) VS. (-T-T0).(2) In a finite plate, it was able to determine a minimum size of specimen, which is equivalent to an infinite plate for the given weld heat input, according to the temperature range of a weld thermal cycle to be considered. When they consider the cooling process to 500 and 300°C, the Value of T in the following formula should be chosen as, T*≤320 for 500°C and T*≤210 for. 300°C, respectively.Where T*=0.24ηJl/mc+T0(°C)η: thermal efficiency of arc, j: heat input (Joule/cm)l: bead length (cm), m: weight of plate (gr) andc: specific heat (Cal/gr.°C)(3) If thickness (t), initial temperature (T0) of plates and weld heat imput (J) are given in the case of single bead-welding, cooling time (s) from 800 to 300°C at manual arc welding and submerged arc welding can be estimated with the following experimental formula or nomographs by the present authors;S=K·Jn/(40-T0)2{1+2/πtan-1(t-t0/a)}

https://www.jstage.jst.go.jp/article/qjjws1943/34/10/34_10_1064/_pdf

Studies of Cooling Processes in the Cases of Welding with Coated Electrode and Submerged Arc Welding

Welding Cracks and Notch-Toughness of Heat-Affected Zone in High-Strength Steels

The nitrogen content of arc-melted liquid iron has been measured in the atmosphere of argon containing nitrogen with various partial pressures. The arc melting was carried out in a small vessel, in which the controlled gas mixture flowed, under the condition of an arc-current of 180 A and straight polarity for the non-consumable tungsten electrode. By a preliminary experiment, the temperature of the molten pool was measured as about 1, 800°C by using W-W.Re thermocuple.The results obtained were as follows:(1) The nitrogen content of the arc-melted pure iron is proportional to the square root of the partial pressure of nitrogen up to about 0.09 atm PN2, and above 0.09 atm the nitrogen content becomes a nearly constant value.(2) In the range of the nitrogen partial pressure lower than 0.09 atm, the nitrogen content of the arc-melted iron is about 5.2 times higher than the value expected from the solubility of nitrogen in the ordinary, non-arc melting. Such anomalous high dissolution of nitrogen may be caused by the activation of nitrogen in the atmosphere by arc.(3) From the difference of the nitrogen content in iron between the arc melting and the non-arc melting, the free energy difference 4F40 between the activated and the ordinary nitrogen is obtained to be 6.82 Kcal/g·atm at 1, 800C. If it is assumed that the activation of nitrogen is essentially the dissociation of the nitrogen molecule to the atomic nitrogen, the above energy difference corresponds to the dissociation constant of 0.15.(4) Calculating the dissociation equilibrium of nitrogen from its dissociation energy and the specific heats of each species, it is shown that the dissociation constant reaches 0.15 at 5, 100C. This is the apparent temperature which the arc-atmosphere would reach in the arc-column, if it is assumed that the composition of the nitrogen gas in the vicinity of the metal surface is the same as that in the arc-column.

Michio Inagaki

Papers

Effect of Residual Tensile Stresses on Threshold Level for Fatigue Crack Propagation in Welded Joints of SM50B Steel

Studies of Cooling Processes in the Cases of Welding with Coated Electrode and Submerged Arc Welding

Welding Cracks and Notch-Toughness of Heat-Affected Zone in High-Strength Steels

Nitrogen Absorption in Arc Melting of Iron

Penetration Depth in Electron Beam Welding (Report 6) : Energy density in beam hole