Quenched and Tempered High-Yield Steel Castings for Offshore Structures
Authors: Schuster F.A. , Hofer Gerhard , Mildner Heinz , SchUtz Hubert 
Published: October 2008

ABSTRACTS

Quenched and tempered plates with a yield strength of 420-500 MPa are currently used in offshore platform fabrication. In this paper, the development of a cast steel is reported, which is fully compatible to quenched and tempered plate materials. Mechanical properties as well as fracture mechanics characteristics and extensive welding procedure qualifications are presented. High strength steel castings offer the chance to manufacture complex heavy-lift and fatigue-critical components for largeroffshore structures without increasing the weight of the platforms.

INTRODUCTION

The use of high integrity steel castings in offshore structural and lifting applications is well established l,2 Deep-water platforms are exposed to severe environmental loads in the form of wind, waves and currents. For these high stressed applications, high strength low alloy (HSLA) steel castings with a yield strength of min. 450 MPa and excellent weldability are essential. An interesting alternative to the normalized steel castings generally used up to now are water quenched and tempered steel casting grades.

In this paper, the aim is to emphasize the effect of modern steel production, improvements in casting design, using computer aided simulation of solidification and stress analysis, on weight reduction. By means of optimum designed steel castings, the high-quality standard of the offshore industry can be maintained.

CAST STEEL DEVELOPMENT

Among the possibilities to improve the toughness and strength of weldable cast steel are:Modifications of the chemical composition of the steelImplementation of modern steel melting techniquesHigher cooling rates during heat treatment

Chemical Composition

To assure good weldability under shop, as well as field conditions, the carbon equivalent should be as low as possible. Old fashioned weldable cast steel grades, for example GS-20Mn5, generally show a carbon level of 0.20 %. For the new steel grades the carbon level has to be reduced to about 0.10 %. The carbon equivalent CE (IIW) is within the range of 0.400.46 %. Some typical chemical compositions ofweldable steel castings and plates are summarized in table 1(3,4,5,6,7). These steel grades are basically MnNiMoCu-alloyed. Niobium and/or vanadium are added primarily to provide grain refinement. In order to avoid severe precipitation hardening effects, V+Nb should be limited to ? 0.10 %. The chemical composition of the modified "GS-13 MnNi 6 4" had to be adapted (as far as possible) to those of quenched and tempered plate materials, Due to the great differences in the wall thickness of steel castings, specifications concerning the Ni-level have been changed to wider ranges.

STEELMAKING

The implementation of modern steel melting technology in the foundry was an important decision in achieving high-strength cast steels with adequate toughness as well as sufficient safety against brittle fracture. The concept of modern steel meltingtechnology is illustrated in fig. 1. Due to the significant influence of sulphur on the toughness of structural steel castings, desulphurisation should already be performed on the hot metal.