Weld Root Embrittlement in Offshore Structural Steels
Authors: Marshall V.G. , Nelson S.G. , Shipley T.D. 
Published: October 2008

It has been recognised for some years that the root region of welded joints presents an obstacle to the achievement of satisfactory charpy and COD-values in the weld metal and HAZ. Furthermore, it has recently become apparent that the problem may be even more significant, in that the zone of poor root toughness can extend into the parent material beyond the visible HAZ (e.g. fusion line + 10 mm), if the joint is subjected to significant distortional strain during welding.

It has been proposed by a number of researchers that root toughness problems are associated with some form of strain ageing embrittlement. However, no detailed analysis of the precise source, magnitude and nature of the stresses involved has yet been conducted.

This paper describes an experimental programme conducted to investigate the link between the metallurgical phenomenon of root toughness degradation and, the stresses which generate the problem. Welded test plates and a finite element analysis model of the root region of the test plates have been used for the investigation.

INTRODUCTION

The demand for higher levels of toughness in structural steels for North Sea production platforms has led to the development of very low sulphur, reduced carbon steels micro alloyed with niobium.

These steels, produced in bulk by either the electric arc or basic oxygen converter process are now subject to the addition of nickel and copper in small quantities to maintain strength levels with reduced carbon content. Control of the aluminium and nitrogen contents are exercised through specification limitations to minimise strain ageing response after cold forming.

The continuous casting process has largely replaced ingot casting for plates up to 75 mm thickness. Improvements in desulphurisation and chemistry control have led to the production of structural steels which exhibit excellent toughness (100 J+), down to minus 60°C, in both the longitudinal and transverse sampling direction.

Welding procedure requirements for North Sea constructions now require that welded joints are tested in the weld, HAZ and parent plate adjacent to the root region. The requirement for testing the parent plate outside the HAZ has produced unexpected problems during both procedure qualification and routine production test plate testing. Material which exhibits charpy impact values of 200 joules at minus 40°C has shown values as low as 15 joules at the root fusion line plus 10 mm position after welding.

The phenomenon is surprising in view of the distance of the degraded area from the visible HAZ. Furthermore, the problem appeared most frequently on tee butt weld configurations, although some fabricators have also reported the same problem on flat plate welds.

A previous programme of work was carried out on submerged arc welds, and is briefly described in the next section. This showed that the problem was associated with test plate restraint, and that the mechanism for the deterioration was probably strain ageing. The results obtained in this work were from plates which exhibited centre line segregation associated with continuously cast material.