Steel Grades io the Yield Strength Range 350 - 550 N/MM² With Improve Weldability or Offshore Structures
Authors: Harvig Hans 
Published: October 2008

INTRODUCTION

Offshore structures provide one of the critical areas of applications for structural steel plates. The severe requirements result in steel quality needs representing the most advanced development in steel plate manufacturing of today. The following material properties are demanded:highest possible yield strength in order to reduce steel plate thicknesses giving productions in weight and less amount of welding,various toughness requirements, the most severe up to now being Charpy-V, transverse min 27 Joules at -50° C, andresistance to hydrogen cracking in order to minimize the need for preheat and interpass heat operations.

It is now common practice to use some type of carbon equivalent to .express the susceptibility to hydrogen cracking. For instance, the IIW formula is often used:(Equation available in full paper)

For plate thicknesses in the range 25 to 50 mm CE ?0.43 is an often specified value for the DIN 17100 St 52–3-type grades. However, it should be pointed out that the carbon equivalent is applicable only for restricted analysis intervals. Thus, it is not a perfect criterion in all situations, for instance, when considering steels with low carbon contents. It should also be remembered that the risk of hydrogen cracking not only depends on the carbon equivalent, but also on the degree of restraint, the cooling rate at the weldment, and the amount of hydrogen available during welding. (d) Resistance to lamellar tearing. In offshore structures the node joints are critical since the restraint normally is great and uncontrolled and the risk of lamellar tearing is Obvious. To detect 'the susceptibility to lamellar tearing of a plate, the reduction of area of tensile test pieces taken in the short transverse direction is the most relevant measure.1 In offshore steel specifications, short transverse reduction of area values from 20 percent (mean of six tests) up to 35 percent are required.

In this paper, the purpose has been to discuss steels on the basis of the above listed requirements with special emphasis on the possibilities of achieving increased resistance to hydrogen cracking. It is, thereby, important to state that the conventional normalized C-Mn-AL- Nb steels are not suitable for higher yield strengths than around 350 N/sq mm since the carbon equivalents then become too high (see Fig. 1). It is also evident that the impact as well as the through thickness requirements are not achievable without special attention being paid to the sulphur and oxygen contents of the steel. Those should be lower than normally specified for conventional St 52-3-type steels.

CRITERION FOR A HIGH-8TRENGTH STEEL WITH IMPROVED RESISTANCE TO HYDROGEN CRACKING

It is, of course, not possible to give a simple definition of "a steel with improved resistance to hydrogen cracking." This is connected with the fact that not only the steel analysis, but also the restraint, cooling conditions in the weldment and the hydrogen content in the weld deposit have influence. In offshore structures the restraint level is often very high. The cooling rates of the weldments are also to be considered as being high since the plate thicknesses are normally large and fillet welds are often frequent.