To improve the in-service behaviour of Cr Mo (V) steel grades used for the pressure vessels operating in hydrogen environment at high temperature for the oil industry, the manufacture of heavy forgings needs a high quality. Improvement of the standard and enhanced strength (ASME Case 1960) 2 l/4 Cr 1 Mo steel grades may be achieved by reducing drastically the impurities (S, P, X, etc…) to extra low level and avoiding segregates at the inner surface of the shells. For high temperature operation, new V modified steel grades are proposed (ASME case 1961, Code Case 1973 and Code Case 2098). Their conventional mechanical properties are similar to those of enhanced strength 2 1/4 Cr 1 Mo but they offer higher creep properties and improved resistance to hydrogen damage.
In the last decade, the evolution and control of steelmaking practice have enabled drastic reductions to be made in the impurity content of low alloy steels (mainly S, P, 0, N and H) and the resulting superclean steel products have seen their in-service behaviour greatly improved. The following examples concerning CLI products are presented : 1 -The reduction of S content by a factor of more than ten and the control of 0 content have considerably reduced the Hydrogen Induced Cracking (HIC) sensitivity of the C-Mn steel grades for the storage of oil and gas in sour environments. 2 -Superclean Ni Cr Mo V steel grades for turbine rotors shafts have a very low sensitivity to lower nose temper embrittlement. 3 -Thick ( z 100 mm) plates or forgings in Mn Ni Mo steel m e used for nuclear pressure vessels welded by the electron beam process offer high toughness in fused areas when S + P are reduced to less than 30 ppm. 4 -Steel grades very close to pure iron have been developed to improve the performance of magnetic circuits in large accelerators.
In order to reduce the number of welds (in service inspection costs), to improve the thermal ageing resistance and the internal soundness of components, Electricité de France decided to develop a solution for forged austenitic stainless steel in place of castings to manufacture the straight parts and the elbows of the primary piping of PWR power plants. The paper presents the interest of such solution and the manufacture conditions of such for-gings applied, first, to a cold leg for the Civaux 1 unit and, second, to a complete loop for the Civaux 2 unit.
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