TWI Technology Briefing 624 - September 1997
R N Gunn and C S Wiesner
FULL REPORT
Duplex and superduplex stainless steels have become common engineering materials, and are utilised generally in the as-welded condition. Over the years, the toughness of welded joints have been assessed and optimised, but there has been uncertainty over the correlation between fracture toughness requirements and the conventional weld procedure qualification (WPQ) tests, such as Charpy V notch. In consequence, impact energy requirements have frequently been set at conservatively high levels, which may be difficult to meet with normal fabrication practice.
Background
Recent Belgian work concluded that achievement of an average absorbed energy of 35J would normally afford sufficient safeguard against unstable fracture in service: surface breaking defects with a depth of 3mm and a length of 15mm could be tolerated in weld metal of 40mm thickness, and even larger defects at the fusion line for service temperatures down to -40°C. Analogous studies at TWI recommended that a Charpy energy requirement of 40J at the operating temperature, for weldments up to 50mm thickness, will ensure adequate defect tolerance and could form a basis for toughness requirements to be implemented for these steels in fabrication codes.
Duplex stainless steels can be prone to formation of embrittling intermetallic phases. given the wrong heat treatment or inadequate weld procedure. Previous TWI work studied these effects and concluded that even small intermetallic contents could have a dramatic effect on toughness. However, the welds described in the Belgian work were made under controlled conditions, consistent with good industrial practice, and were not expected to contain intermetallic precipitates. The TWI studies considered the effects of intermetallic precipitates on weld metal toughness and reached the conclusion that the Charpy/CTOD correlation was not affected by the presence of intermetallic phases up to about 3% volume fraction. For the Charpy/CTOD correlations established to be of general applicability, it is necessary to ensure that intermetallic phases have a similar effect on toughness criteria for parent steel and HAZ microstructures.
In addition, an increased database would enhance confidence in the previous findings. Accordingly, the aim of the current work was to evaluate the influence of different intermetallic contents in a superduplex parent steel on CTOD and Charpy V notch tests.
Objective
- To evaluate the effect of intermetallic precipitation on both fracture toughness and impact test results in parent steel and HAZs of superduplex stainless steel.
Experimental approach
Samples were removed from 219mm outside diameter 12.7mm wall thickness pipe to UNS S32750 in the axial direction. These samples were subject to a matrix of heat treatment, viz: as-received pipe, re-solution annealed, HAZ simulation, and 'sigmatised' by heat treatment at 1000°C for 5min.
Sections from the samples were examined metallographically, and the intermetallic content determined by point counting. They were further examined under a scanning electron microscope (SEM), and, in order to facilitate identification of intermetallic phases, imaging was undertaken in the back-scattered mode. Charpy V notch impact and CTOD fracture toughness tests were conducted for each heat treatment type at temperatures in the range -90 to 0°C.
Results and discussion
Comparison of the intermetallic contents in the four sample types showed that the range covered 0-8% volume fraction, ie:
- solution annealed (none)
- as-received (<0.01%)
- thermal simulation (0.09%)
- sigmatised (8%) conditions.
Toughness tests showed a marked reduction in both Charpy and CTOD behaviour with increase in intermetallic content, especially at the 8% level.
There was good correlation between the results for comparable Charpy and CTOD specimens ( ie same heat treatment and test temperature), irrespective of intermetallic content and test temperature. Further, the current data are comparable with results of earlier work and show that the Charpy/CTOD correlations previously developed hold for parent material and HAZ microstructure containing intermetallic phases. This has practical significance for WPQ tests where toughness requirements are, in general, expressed by Charpy V notch tests and for which existing fabrication codes do not normally allow for the presence of any intermetallic phases.
Examination of the fracture faces from Charpy and CTOD samples revealed similar features in both as-received and sigmatised conditions, for samples of similar toughness, but tested at different temperatures. The implication of the CTOD results is that there was little effect of the intermetallic phase on the global fracture mode, which remained fully ductile. However, the sigmatised samples exhibited a different micromechanism in that cleavage events took place in the ferrite phase at -60°C. This compares to fully ductile behaviour for as-received material.
Main conclusions
- There was a marked drop in toughness properties with increased intermetallic content. The effect of intermetallic phases on toughness was to cause local cleavage events in the ferrite phase only, rather than changing the globalfracture mode which remained fully ductile.
- Similar behaviour was observed for both Charpy V notch and CTOD tests with regard to intermetallic phase content and test temperature for the 10mm specimen thickness studied.
- A unique correlation was determined between Charpy and CTOD toughness results, irrespective of intermetallic content and test temperature (for 10mm thick specimens).
Recommendations
Other work has concluded that an appropriate Charpy requirement for duplex and superduplex steel is 40J for typical weldment flaw dimensions at the minimum operating temperature. The current work has validated this conclusion for parent materials and HAZs containing up to 8% intermetallic phase. It should therefore be regarded as applicable to welds in duplex steels containing intermetallic phases up to this level.
Member Report No. 624-1997
Effect of HAZ intermetallic precipitates on the toughness of superduplex stainless steel
