Recently, it became necessary to investigate the weldability, especially the weld cracking on weld joints, of the atmospheric corrosion resistance steels (ACR Steel). A test was, therefore, made to seek for the root cracking on three kinds of 50 kg/mm2 class ACR Steel (36 mm thick) using the y groove weld cracking specimen regarded as the most famous one for the crack sensibility of the base metals in Japan. Assuming that the specimen would have various sizes, it seems that the difference might be caused by the intensity of restraint and the cooling rate of welding in each of the y groove specimens. Four sorts of specimens, the sizes of which were different, were prepared to examine the crack sensibility of the base plate effected by weld heat, and then the correlation of their intensity of restraint was considered. Moreover the comparison between 50 kg/mm2 class ACR Steel and SM 50 plate was mainly carried out in crack sensibility also. The results of experiment are as follows; (1) The crack sensibility confirmed with JIS Z 3158 Specimen (y groove cracking specimen of one it and smaller than Tekken type) was higher than that with the traditional Tekken type specimen (two slits). (2) The restraint coefficient Ko of cracking specimens could be determined by the product of the modified coefficient a depending upon the restraint state of the specimen and the restraint coefficient K concerned with the slit length in the infinite plate. K0=a⋅K (3) Mere was no tendency noticed that the crack sensibility of 50 kg/mm2 class ACR Steel grew worse than that of SM50 plate.
The pulsating tensile fatigue tests have been carried out on the butt welded specimens using a Vibrophore fatigue testing machine. The microscopic examintion of these specimens has revealed the presence of small notches or grooves which have a sharp root radius and exist mostly at the periphery of the deposited metal near the welded toe. In any case, fatigue crack is initiated at these notches which are exceedingly small, not more than 0.03 mm in root radius and less than 0.1 mm in depth. In case of fatigue cracking at the periphery of the deposited metal, the smaller the root radius and the deeper the notch, the shorter becomes the fatigue life. Therefore fatigue strength increases as the periphery of the deposited metal becomes very smooth and the root radius of the nocthes becomes large, even if an undercut may be present. The fact that the fatigue strengths of welded materials with different static strengths show nearly similar values can be explained by the presence of such a sharp notch and not by the marcoscopic notch effect. Besides the marcoscopic stress concentration due to the geometrical effect of the welded toe, another microscopic stress concentration caused by these small notches has been confirmed, and such notches influence the fatigue strength of welded specimen. A conspicuous improvement of the fatigue strength of the welded joints can be attained if the welded toe and the periphery of the deposited metal in the vicinity of the toe are smoothed by some method of welding or post treatment after welding.
In the welded joint, chemical composition and structure of weld metal or heat-affected zone which is adjacent to it are different from those of parent metal. Hence, the mechanical properties, for example stress-strain relation, of the welded joints must also be different from those of parent metal and change continuously. In this way, considering macroscopically, the welded joints are not homogeneous nor isotropic. Concerning such welded joints which are heterogeneous, it matters a great deal to investigate their strength and ductility. The welded joints are fundamentally idealized as models which are joints consisting solely of parent metal and a soft or hard interlayer. By "soft interlayer" is meant apart of welded joint in which yield point is lower than that of parent metal. In the present report, such model specimens were made by flash butt welding. The parent metal was grade S35C, the soft interlayer grade S10C. The specimens were given post-weld heat treatment (850°C×10 min. water quenching and 500°C×30 min. tempering) to cause an abrupt change of the mechanical properties between parent metals and soft interlayer. Static tensile test was made for four kinds of specimen diameter, 3, 6, 10 and 15 mm, at room temperature. In the joints including a soft interlayer under tension, plastic flow will begin at first in the parts of a soft interlayer. The transverse plastic flow of interlayer at the contact surfaces and the neighbouring zone will be held in check by the stronger parent metal, and the interlayer will be in a triaxial stress state analogous to that in the neck of tension specimen. The triaxiality will be severe with a decrease in the thickness of soft interlayer. The ultimate tensile strength of the joints exceeds that of the material of interlayer, if the values of thickness of interlayer are sufficiently low. The present paper describes theoretical and experimental investigations on the effect of thickness of a soft interlayer or specimen diameter on mechanical properties of welded specimens under static tension. The results obtained are as follows. (1) The yeild stress, ultimate tensile strength, plastic constraint factor and reduction in area of the welded joint including a soft interlayer are represented as functions of the relative thickness X, or the ratio of thickness of soft interlayer to specimen diameter, regardless of the specimen diameter. (2) The yield stress and ultimate tensile strength increase and the elongation and reduction in area decrease as the relative thickness decreases. Fracture mode becomes brittle for a sufficiently small value of X, although the material of interlayer possesses high ductility under unidirectional tension at room temperature. (3) The test results on ultimate tensile strength agree with the results of the authors' theoretical analysis for a comparatively large value of X.
The intensity of restraint will be one of the, important factors in weld cracking. It has been recently reported that the critical intensity of restraint Kcr for given materials and welding conditions can be obtained from the RRC-test results and that the occurrence of weld cracks in the actual weld joints may be estimated by the comparison of the Kcr-value with the intensity of restraint K of actual weld joint. Evaluation of the K-value for actual weld joints will be of practical importance. The present paper deals with the evaluation of the K-value for the weld joints as shown in Figs. I (b) (c). In the evaluation were considered elastic and plastic deformations in fillet welds produced by restraint forts which were not taken into consideration in the investigation by Terao, et al.. Theoretical analysis revealed that the K-values are influenced by the factors such as the size of fillet weld or total throat depth a, welding heat input Q and uncompleted weld length 2 d. The K-value decreasee with an increase of Q (See Figs. 7thru9.) and increases with an increase of a and a decrease of d. No remarkable increase of the K-value is observed with a decrease of G as the result of plastic deformations of fillet welds. Examples calculated for sevral conditions are shown in Figs. 11, 12, 14.
This paper is concerned with the weldability of spot welding nickel, copper, chromium and tin plated steel in 1 mm thickness. The apropriate welding condition range, mechanical properties, hardness of the weld nugget, nugget shape and the voltage drop across electrodes are studied and the optimum welding conditions are determined for each plated steel. As the result of investigation, the following are concluded. (1) Welding condition is greatly dependent of the kind of metal plated on steel. Higher welding current is required for the tin and the copper plated steel and lower current for the chromium plated steel comparing with the bare steel. High electrode force is recommended for spot welding of the chromium plated steel. The time necessary to produce a nugget is longer in the case of plated steels than of the bare steel and consequently longer weld time is recommended for spot welding plated steels. (2) When electrodes with the same shape and dimensions are used, tensile shear strengths of plated steels are nearly equal to that of the bare steel, but cross tension strengths of the nickel and the chromium plated steel are considerably low. (3) The welding conditions depend a little on the plating thickness over the range from 2.5 to 10 p in thickness. (4) The nuggests of the nickel, copper and chromium plated steels are high in hardness and the hardness of the weld nugget of the tin plated steels is about the same as that of the bare steel.
The present study is an application of the NRIM TRC test (Tensile Restraint Cracking test) to cold cracking in multilayer welds of a high-sterngth steel with a strength level of 100 kg/mm2. It was aimed in this study to make clear the cracking behavior and to get practical methods for the prevention of cracking. A TRC specimen was set up in a 1000 tons NRIM TRC tester and pulled after finishing of a test weld. The constant loading at various stress levels was sustained for sufficient time for cracking. There occurred two types of macrocracks in multilayer welds depending on the direction of restraining, that is, transverse and longitudinal cracks in longitudinal and transverse TRC tests, respectively. Those cracks in longitudinal and transverse TRC tests, respectively. Those cracks were considered extensions of microcracks. Cracks, both transverse and longitudinal, were the delayed type due to hydrogen and occurred after an incubation period under a sustained tensile loading which exceeded the critical tensile stress for cracking. The incubation period extended from about 1 to 10 hr, mainly depending on restraining stress and hydrogen content. The critical tensile stress for cracking was raised with an increase of preheating temperature and with a decrease of hydrogen content. The critical tensile stress for longitudinal cracking was slightly lower than that for transverse cracking at the same welding conditions and hydrogen content. Transverse cracks were prevented under the longitudinal tensile stress of 65 kg/mm2 with any of the following methods: (a) preheating at 200°C, (b) 100°C preheating and locally tempering postheating at 500°C, or (c) 150°C preheating and lower temperature postheating for 5 hr at 100°C. The restraining stress of 65 kg/mm2 was a little over the restraining stress in the so-called window restrained test which has been appreciated as fairly reporducible one of actual severe restraint. Very little change of hydrogen content, about 0.2 cc/ 100 g, in deposited metal was decisive to transverse cracking in such high strength steel welds, supposing actual joints with severe restraint. Longitudinal cracks were gradually propagated and then usually caused a brittle fracture throughout the joint. The carck was prevented with preheating at 200°C under the transverse tensile stress of 55 kg/mm2 At the tensile stress of 65 kg/mm2 any crack did not occur with simultaneous use of 150°Cpreheating and lower temperature postheating, 200°C for 1 hr or 100°C for 5 hr. Effectiveness of lower temperature postheating was discussed with an estimation of existing hydrogen content in a weld at the instant of cracking.
In the previous reports. Author indicated that preheating flame has the function to keep purityy and momentum of cutting oxygen stream. It may be supposed that the function of this kind of preheating flame should give an influence on the material thickness to be cut. Experiment was done about relation between maximum cutting thickness and flame position to cutting oxygen stream using several types of cutting torch, and following results, were obtained. 1) The function of gas flame to keep momentum of oxygen stream was reconfirmed, and the keeping effect was found to be related to flame position to oxygen stream. 2) The momentum of oxygen stream has close relation to maximum cutting thickness (Tm) and therefore preheating flame affects cutting thickness. 3) Position of flame to oxygen stream has relation to Tm-log ν characteristics. 4) Cutting of thinner material is affected by oxygen purity and a thicker material affected by oxygen momentum. 5) Some other functions of preheating flame will be made clear by introducing the keeping effect of, oxygen purity and momentum in analizing the cutting phenomena.