Welding of Carbon and Low Alloy Steels and Hydrogen Induced Cracking Sep23

Welding of Carbon and Low Alloy Steels and Hydrogen Induced Cracking...

Welding of Carbon and Low Alloy Steels and Hydrogen Induced Cracking Arc welding is a process by which steels are joined by coalescence. Normally the process uses a compatible filler material. Before a well-bonded joint is produced, the joint surface is to be heated above the melting temperature in order to completely fuse with the weld metal. Though the metallurgical reactions which involve melting, solidification, and solid-state transformation are not unusual, the temperatures and cooling rates observed are severe. Active gases also are present and can dissolve in the fused steel. Fluxes are introduced to alloy with and protect the weld metal. Generally, joints are rigid and restrain dimensional changes caused by shrinkage and solid-state transformations, producing residual stresses of yield-strength (YS) magnitude. Since the metallurgical changes do not occur under equilibrium conditions, and since the stresses are high, many of the reactions can take place in either or both the weld metal and the heat affected zone (HAZ) of the steel and can produce defects that weaken their soundness. Because of the tremendous variability of the welding processes, it is difficult to provide much detail about the exact mechanisms involved or the corrections that can be made. Furthermore, many corrective measures are obvious once most defects are explained. One problem, which relates to hydrogen (H2), is not simple. Since this problem is becoming more relevant as more high-strength, low-alloy (HSLA) steels are being welded, the subject of hydrogen-induced cracking (HIC) is very important. Carbon (C) and low alloy steels are welded since they have widespread application and good weldability. This usefulness is mainly due to the metallurgical characteristics of the iron (Fe) base system. The characteristic includes the ability to undergo allotropic (microstructural) transformation which allows the opportunity for hardening and strengthening through...

Welding Processes Apr10

Welding Processes

Welding Processes Welding is a fabrication process that joins materials by causing coalescence. Welding is normally carried out by meltingĀ the work pieces and adding a filler material to form a pool of molten material that cools to become a strong joint, either with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with solderingĀ and brazing, which involve melting a lower melting point material between the work pieces to form a bond between them, without melting the work pieces. Welding usually requires a heat source to produce a high temperature zone to melt the material, though it is possible to weld two metal pieces without much increase in temperature. There are some methods with solid phase joining. In these methods there is no melting of the electrodes, though heat is produced in the process. Also since the work pieces are closely pressed together, air is excluded during the joining process. In normal welding the melted and solidified material is normally weaker than the wrought material of the same composition. In the solid phase joining such melting does not occur and hence the method can produce joints of high quality. Metals which are dissimilar in nature can also be readily welded by these methods. In the normal welding process, joining of dissimilar metals presents problems since brittle intermetallic compounds are formed during melting. Modern welding technology started just before the end of the 19th century with the development of methods for generating high temperature in localized zones. There are different methods and standards adopted and there is still a continuous search for new and improved methods of welding. Though the different welding processes have their own advantages and limitations and are required for special and specific applications,...