Riser girth weld toughness - an HAZ microstructure issue

TWI Bulletin, May - June 2004

Just how important is our understanding of the heat affected zone's microstructure for deducing the toughness of weld metal in safety critical parts?

Henryk Pisarski is Principal Consultant in the Structural Integrity Group at TWI. He is concerned with application of fracture mechanics testing and flaw assessment procedures to ensure the integrity of welded components. He has contributed to codes and standards and published papers on fracture toughness testing and integrity assessment.

David Abson gained his first degree at Cambridge University and then a Masters degree and a PhD at the University of Sheffield. After working as a post-doctoral research fellow at McGill University Montreal and the University of Wales, and a period of work for Westinghouse in the USA, he joined TWI's Materials Department.

Jack Still worked first in the steel industry as a metallurgist advising customers and then the fabrication industry. During the last 24 years has worked for various companies on projects involving jacket and production modules, FPSO construction and structure refurbishment.

To assess the structural integrity of a gas riser on an offshore platform it was necessary to establish the fracture toughness of girth welds. As Henryk Pisarski, David Abson and Jack Still report removal of specimens from the riser was not possible, so test welds had to be made, simulating the riser girth welds, from which specimens could be prepared.

To obtain heat affected zone (HAZ) fracture toughness data representative of the original riser girth welds, a section of the original 16in riser pipe material was used to produce weld samples in accordance with the welding parameters quoted on the Weld Procedure Operating Instruction (WPOI). The WPOI was developed from the Weld Procedure Qualification Record (WPQR).

The outcome of this exercise identified that the HAZ CTOD results reported were scattered, but subsequent metallography identified a relationship between HAZ microstructure and CTOD value. This emphasized the need to carry out post testing metallography on fracture toughness specimens that tested the HAZ, otherwise misleading conclusions could be drawn with respect to the effect of constraint and temperature on fracture toughness.

Riser pipe materials

The riser pipe with an outside diameter of 16in and wall thickness of 18.6mm was fabricated from API 5L X65 seamless steel pipe, supplied in the quenched and tempered condition. A section of surplus riser pipe manufactured by electric arc steel making process was used for the test weld procedures. Details of the mechanical properties and chemical analyses from the mill test certificates are given in Tables 1 and 2.

Table 1 Mechanical properties and chemical analysis of riser pipe test material

Table 2 Riser pipe test material check analysis

Welding procedures

Although the original WPQR comprised a single V butt weld with an included angle of 60 - 70°, it was decided to obtain a near vertical HAZ using a half-K type preparation, in order to facilitate notch location in the fracture toughness specimens. Three girth welds were made: welds A and A1 were identical, but weld B was made in a wider groove and lower arc energy than the other two.

Details are as follows:

Welds A1 and A2
Weld preparation was made in a 40-50° included angle single bevel butt joint, ie half-K type preparation.
Welding position 5G.
Root run and hot pass - Gas Tungsten Arc Welding (GTAW) at arc energies 1.4-2.4kJ/mm.
Fill - Shielded metal arc welding (SMAW) at arc energies 1.1-2.6kJ/mm.

Weld B
Weld preparation was made in a 60° included angle single bevel butt joint, ie half-K type preparation.
Welding position 5G.
Root run and hot pass GTAW at arc energies 1.0-1.9kJ/mm.
Fill - SMAW at arc energies 1.0-2.1kJ/mm

Hardness traverses

Vickers diamond pyramid hardness tests were conducted on each of the test welds (5kg load) where the hardness of the parent material and weld metal regions were, on average, found to be approximately 200HV5. It is noteworthy that the highest hardness locations within the hardness traverses were frequently found to occur in the HAZ, adjacent to the fusion boundary, where values up to 240HV5 were not uncommon. In one location (the HAZ of macro Section A1, just under the capping layer), a hardness value of 296HV5 was recorded.

This hardness was observed to be associated with a 'bay' region (between two adjacent weld passes) containing MA (martensite-austenite) constituents. This area was associated with the intercritically reheated coarse graind HAZ (ICGCHAZ) due to reheating of the grain coarsened HAZ (GCHAZ). Figure 1 shows the ICGCHAZ regions within the HAZ. Vickers diamond hardness surveys using a 500g Duramin load on test welds A1 and A2 recorded hardness values of 239 - 246HV and 233-239HV, respectively.

Microstructural observations

Examination of the macrosections identified the ICGCHAZ regions adjacent to the straight edge preparation. These regions are small in comparison to the GCHAZ. Nevertheless, they have been observed to contribute to low HAZ toughness in fracture mechanics tests. The locations of ICGCHAZ regions within the macro sections examined are listed in Table 3. It was noted that these regions were often, though not always, found to be present at the mid-thickness location. Metallographic examination of the macro sections of welds A1 and A2 revealed the presence of grain-boundary decorations of an unresolved phase, MA (martensite-austenite), in a region of ICGCHAZ, Fig.2.

Table 3 Location of regions of intercritically reheated grain coarsened HAZ

Fracture mechanics test specimens

Fracture toughness specimens (Bx2B, through-thickness notched, where B is the specimen thickness and in this case similar to the pipe wall thickness) were manufactured and tested according to BS 7448. Forty-four fracture mechanics specimens were prepared from three welds. These were tested at -10°C and + 4°C. The test temperatures were selected on the basis that the riser girth welds were located below or above the splash zone, respectively.

The specimens were tested in the side-grooved condition (with a/W = 0.6) or non-side grooved condition (with a/W = 0.5, where a is the crack depth and W the specimen width, in this case 2B).

Deeply notched, side grooved specimens were employed initially because ductile behaviour was expected, and a resistance curve could be obtained according to BS 7448:Part 4. When brittle fracture was observed, subsequent specimen design was changed to BS 7448:Parts 1 and 2, with a/W = 0.5. The fatigue pre-cracks were targeted to test the HAZ toughness properties. Critical CTOD and stress intensity factor (KJ), derived from J, was determined from each specimen.

Examination of fracture mechanics specimens

Ten fracture mechanics test specimens were selected for post-test fractography and metallographic examination, as listed in Table 4. (Note, the results in Table 4 represent a sample of the specimens tested.) These specimens were examined in order to reveal the microstructure sampled by the fatigue crack tip and the microstructure at fracture initiation; see Table 5. Two specimens fractured after the attainment of a maximum load plateau ,classed as 'm' in Table 4 and the remainder fractured, classed as 'c' or 'u' in Table 4.

Table 4 Fracture mechanics specimens examined

Table 5 Microstructural observations made on the metallographic sections

Photographs of the sectioned fracture toughness specimens and the microstructure at the initiation region, are presented in Fig.3, 4 and 5. In each of these figures, fracture initiation is in the centre of the fracture edge shown. It is noteworthy that a number of specimens tested showed significant amounts of weld metal over the length of the fatigue pre-crack. This was attributed to the sensitivity of vertical fusion boundaries to notch location ( eg small variations in notch location can result in large differences in the micro-structure sampled at the crack tip), and also as a consequence of a non-planar fusion boundary (at the microscale).

Fig.3. Test weld A CTOD specimen fracture initiation regions

Macro specimen	Microstructure initiation region	Microstructure sampled by the fatigue pre-crack	Brittle fracture initiation location
		100% Weld metal	Reheated weld metal. Approximately 1 mm from Fusion Boundary of a subsequent pass Fracture Initiation Regions Near bottom of fracture edge
		33% GCHAZ (Coarse grained HAZ) 67% FGHAZ (Fine grained HAZ Near top of fracture edge	Inter-critically reheated GCHAZ Fracture Initiation Regions Near top of fracture edge.

Fig.4. Test weld A1 CTOD specimen fracture initiation regions

Macro specimen	Microstructure initiation region	Microstructures sampled by the fatigue pre-crack	Brittle fracture initiation location
		Mostly weld metal Small amount of HAZ, some GCHAZ	ICGCHAZ Initiation in a small 'bay' region beside the overlap of two weld beads Fracture Initiation Region Near top of fracture edge.
		12% FGHAZ 12% GCHAZ (0.5 mm from Fusion Boundary)	GCHAZ Fracture Initiation Region Near top of fracture edge.

Fig.5. Test weld B CTOD specimen fracture initiation regions

Macro specimen	Microstructure of initiation region	Microstructure sampled by the fatigue pre-crack	Brittle fracture initiation location
		67% fusion boundary small patch of sub-critically reheated HAZ	ICGCHAZ Initiation in a small 'bay' region beside the overlap of two weld beads Fracture Initiation Region Near top of fracture edge
		50% Fusion boundary + HAZ Pre-crack 0.5 mm from FB in HAZ Approx 1.7 mm of GCHAZ sampled 50% Weld metal	CGCHAZ Initiation in small 'bay' region beside the overlap of two weld beads Fracture Initiation Region Near centre of fracture edge

A number of the specimens were found to have sampled large amounts of the fine grained HAZ. This effect was attributed primarily to the substantial grain refinement produced during welding, owing to weld bead placement adjacent to the vertical face of the preparation, and secondly to the position of the fatigue pre-crack.

It was established that the extensive grain refinement present in the weld joints limited the amount of grain coarsened HAZ that could be sampled. Nevertheless, initiation of the fracture had occurred in the grain coarsened HAZ in five specimens from the test welds. Of these, the three that exhibited the lowest fracture toughness values were all found to have initiated in a region of ICGCHAZ. The ICGCHAZ contained MA phases decorating the prior austenite grain boundaries.

Nevertheless in Fig 2, it will be noted that there are (transgranular) cleavage facets at the fracture edge. Thus, any reduction in fracture toughness associated with MA has not given rise to intergranular fracture. However, the MA constituents may prevent grain boundary accommodation of deformation processes within large prior-austenite grains, making cleavage fracture more likely in such grains.

Conclusions

Post-test metallography established that the grain coarsened HAZ region was, more often than not, the site for brittle fracture initiation. Furthermore, brittle fracture initiation was, in a number of cases, found to be associated with regions of intercritically reheated grain coarsened HAZ. Detailed examination of a number of these regions revealed the presence of decorations (micro-phases) along the grain boundaries, and in some cases these decorations were found to consist of MA (martensite-austenite), which are associated with low fracture toughness values.

The HAZ fracture toughness results appear to show that the effect of an increase in the test temperature and a decrease in the constraint level (due to changes in specimen geometry ie a/W reduced from 0.6 to 0.5 and no side grooves) is to increase fracture toughness. However, post-test metallography showed that the reason for the increase in toughness was really due to the fact that the specimens sampled tough fine grain HAZ or weld metal and not brittle ICGCHAZ or GCHAZ. Consequently, for the purposes of the integrity assessment it was necessary to assume (conservatively) the same fracture toughness for welds below the splash zone as those above the splash zone ( ie the results obtained at -10°C).