FOR INTERNAL USE ONLY
Figure 35
Correlation of HAZ Hardness and CE as a Function of Cooling Rate
CE’s are used widely in industry as measures of weldability. Several different formulae have been developed and some are even incorporated into national codes and specifications. In general terms, other factors being equal, as the carbon content increases, so does the difficulty in weldability. In practice, this means generally using higher preheats until cracking and restraint problems are overcome. Using an engineering/analytical approach becomes very useful when confronted with unknown material compositions, and weld repairs can become challenging where reverse engineering must be utilized to develop a repair procedure. The engineering approach may involve evaluating composition, hardenability, service conditions, size, restraint conditions, and PWHT feasibility. One of the popular methods for determining weldability is to review the hardenability of the base material. As discussed earlier the CE formula(s) have been developed as a convenient method of normalizing the chemical composition of a material into a single number to indicate its hardenability. Review of the literature indicates no less than a dozen different formulas have been developed. One of the most commonly used formulas for calculating the CE is the IIW formula (shown in Figure 36): CE = C +
Mn Cr + Mo + V Ni + Cu + + 6 5 15
It must be stated that low carbon steel and carbon – manganese steels generally behave in a predictable manner and are successfully welded with preheat and PWHT criteria outlined in codes such as AWS D1.1, Structural Welding Code – Steel. The CE is not usually evaluated on these materials. Medium carbon, HSLA, and Q&T Steels, however, present different challenges where consideration of CE, restraint, hydrogen control, PWHT not practicable, weld filler chemistry mismatch, weld heat input etc. can be critical to successful repair welding. These factors can be summed up as a materials weldability, and it is these factors that will be considered in Section 8.
5.2
TEMPERING – EFFECTS OF REHEATING
As discussed earlier martensite produced in a quenched steel is hard and brittle and in most cases the steel is unusable in that form. The toughness may be improved by a process of tempering. This involves reheating the steel to below the transformation temperature (723°C), holding for a period of time, then cooling to ambient temperature as illustrated in Figure 37. During tempering the carbon trapped as an interstitial in the martensitic tetragonal structure is released. Carbon atoms diffuse and precipitate as small carbides. With enough time and at sufficiently high temperatures cementite (Fe3C) forms, not as plates as in pearlite, but as spherical particles. This microstructure is known as bainite(see section 3.9).
WELDING AND COATING METALLURGY2
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