Hydrogen Cracking of Ferritic-Austenitic Stainless Steel Weld Metal

Manuscript Number: P2058

Presented at 'Stainless Steel World Duplex America 2000' conference, 29 February - 1 March 2000

Abstract

First, a procedure was derived for hydrogen analysis of ferritic-austenitic SMA weld metals, based on the vacuum hot extraction method. Second, self-retrained single run and multipass weld cracking tests were performed, with consumables varying in deposit hydrogen potential and phase balance. Weld metal cracking was found to occur only at high ferrite contents and hydrogen levels. The results were correlated with other published information, and a diagram was produced relating cracking to the consumable hydrogen content determined under standard conditions and the deposit phase balance. From this correlation, the risk of hydrogen cracking in practice is expected to be low, given adoption of normal welding procedure guidelines.

1. Introduction

Conventionally, hydrogen cracking risk with transformable ferritic steels is dependent upon the diffusible hydrogen content at normal ambient temperature, and this can be determined using a standard procedure with collection over mercury ^[8] . With duplex stainless steels, overall diffusion rates are slow because of the presence of austenite ^[5] , although local diffusion in ferrite may well be as rapid as with carbon-manganese steels. Thus, collection of diffusible hydrogen at room temperature is impractical, and a number of studies have recommended extraction at elevated temperatures ^{[1-7,] [9, 10]} . However, there remains a need for definition and standardisation of a suitable analysis temperature and time.

The present work was in two phases. First, a procedure was derived for hydrogen analysis of ferritic-austenitic SMA weld metals based on the vacuum hot extraction method (VHE) ^[11] . Not all laboratories have access to VHE equipment for measuring hydrogen in a production situation. Therefore, weld metal hydrogen levels were also measured using vacuum encapsulation of a sample in quartz glass ^[3] followed by heating for hydrogen extraction and analysis by a commercial gas chromatography instrument. The aim was to identify an extraction temperature that would give consistent and complete hydrogen totals within a reasonable time period, i.e. less than 24 hours, which would allow rapid quality control of electrode batches. Second, self-restrained weld metal cracking tests were carried out, involving both single run and multipass assemblies, to examine the conditions of hydrogen content and deposit microstructure under which the problem was likely to occur.

2. Experimental Procedure

2.1 Materials

Table 1 Compositions of materials employed in the study

Duplex stainless steel plate, grade UNS S31803, was used for both single and multipass welds in Phase II. Plates of 12 and 20mm thickness were used for the single pass and multipass deposits respectively. The single pass welds utilised two SMA consumables. A standard 22%Cr duplex consumable was employed, together with an experimental electrode, designed to produce high weld metal ferrite levels (~85%). Both consumables were 3.2mm in diameter and had a rutile flux coating. The multipass welds studied in Phase II utilised three consumables. Two SMA consumables were used, viz a commercial 25%Cr electrode, and an experimental 22%Cr electrode with only 5.3%Ni to give a high ferrite level (62-85%), while a third weld was produced using a commercial 22%Cr flux cored wire.

2.2 Derivation of Hydrogen Analysis Procedure

Table 2 Materials employed and welding conditions used for Phase I deposits.
(a) Evaluating hydrogen behaviour.

(b) Varying consumable type and welding process.

On completion, most of the test pieces were stored in liquid nitrogen until analysis was performed. However, one sample of a triplicate set was left under ambient conditions for four days, to study the need for liquid nitrogen storage. The hydrogen evolution behaviour of each deposit, in terms of ml/H ₂ /STP/100g deposited weld metal, was determined against time by the VHE method at different temperatures. This was generally based on individual deposits, although duplicate or triplicate analyses were carried out on selected samples to investigate the level of repeatability in the VHE analysis. If extraction required longer than six hours, the specimens were allowed to cool and then reheated under vacuum the next day. The high austenite deposit 25/80 ( Table 2) was first analysed at extraction temperatures over the range 400°C to 950°C, and the time required for hydrogen evolution was noted. Once the temperature that would lead to hydrogen analysis results in less than one day was identified, the evolution behaviour of two duplex deposits (i.e. 22/60 and 25/60 in Table 2) was explored. Analysis was then carried out using other SMA electrodes and submerged arc and cored wire welding, as in Tables 1 and 2, using triplicate samples.

Comparison between VHE and encapsulation methods of hydrogen analysis was made using three commercially available 22Cr duplex stainless steel consumables, the analyses of which are included in Table 1. All of the consumables were deposited on type S31803 plate. VHE was carried out on each of the deposits at a temperature of 700°C for 3.5 hours. Vacuum encapsulated samples were heated in a furnace at 400°C for 72 hours to allow for hydrogen evolution: the hydrogen content was then determined by gas chromatography (GC) at ambient temperature by breaking the capsule in a flow of Ar carrier gas using a Yanaco G-1006 instrument. In the event, the results obtained indicated that complete extraction into the capsule was not being achieved. Accordingly, an additional test was undertaken with titanium powder added to the capsule to getter any hydrogen evolved from the sample.

2.3 Hydrogen Cracking Tests

2.3.1 Single Run Welds

To obtain different hydrogen levels, test electrodes of each type were placed in a humidity cabinet, at 28.5°C and 74% humidity, for 24 hours, or baked at 350°C for 2 hours prior to welding. Single run, Y-groove test welds were deposited using each electrode type and hydrogen potential. Welding was carried out at 22°C and 46% relative humidity. A total of four test welds was deposited, the welding conditions again being chosen to give an aim arc energy of 1kJ/mm ( Table 3).

Table 3 Welding conditions used in the Phase II Y-groove cracking tests

Hydrogen analysis test blocks were machined from the same test plate as the Y-groove samples in accordance with BS 6693 ^[13] . Triplicate sets of blocks were machined for each electrode and aim hydrogen level. Test electrodes were either baked or humidified as described above. Test weld beads were deposited at the same time as the Y-groove test runs to ensure similar testing conditions. All test runs were deposited with arc energies of ~1kJ/mm. After welding, the test blocks were cleaned to remove the slag and were stored in liquid nitrogen. The hydrogen content was then determined by VHE at 950°C for 1.5 hours.

On completion of welding, the Y-groove samples were left for 1 week prior to examination. After this time, the weld caps were examined for evidence of any surface cracks. In addition, each test weld was sectioned as detailed in JIS Z3158 ^[12] , which requires a total of five transverse sections to be taken through the weld bead at equal distances apart, and examined for evidence of cracking.

2.3.2 Cracking in Multipass Welds

Table 4 Summary of Phase II - Multipass welding conditions

As soon as each panel cooled from welding, it was inspected using 45° and 70° angled compression wave probes, from all three orthogonal directions, including the weld root. The equipment employed was a Force Institute P-scan system with a sensitivity set using a 3mm diameter side drilled hole in a duplex test block, following BS 3923 part 1 ^[14] . This system takes ultrasonic (UT) signals and probe positions to produce a three view image of the weld volume with a detection limit of about 0.5mm. Subsequent UT inspection of each panel took place after 1, 2, 4, 8, 16 and 30 days, and then monthly, until at least six months had expired. For comparison, dye penetrant inspection was undertaken, as was radiography using 400keV X-ray equipment viewed from the cap side.

2.4 Metallography

3. Results

3.1 Derivation of Hydrogen Analysis Procedure

3.1.1 Hydrogen Evolution

Comparison of the evolution behaviour at 700°C for different deposits and base materials is given in Fig.2

Results of the analyses on a range of deposits are given in Table 6

Table 6 Hydrogen contents of commercial consumables

The sample stored under ambient conditions prior to analysis gave a result of 19.3ml of H 2

3.1.2 Comparison between VHE and encapsulation techniques

Table 7 Summary of results for different hydrogen analysis techniques and sample types

3.2.1 Y-Groove Tests

Table 8 Weld metal hydrogen results - Y-groove cracking test samples.

Table 9 Weld metal ferrite content and Y-groove cracking test results.

The weld metal microstructures were typical of duplex stainless steel deposits. The high ferrite welds contained some grain boundary and isolated intragranular austenite. Cracking in each of the high ferrite welds was transgranular in nature and propagated through the ferrite phase, Fig.3

Table 10 Multipass weld metal ferrite and hydrogen content.

4.1 Hydrogen Analysis Procedure

Table 11 Recommended minimum extraction times for VHE.

Table 6

The results of this work clearly indicate that the encapsulation method gives lower readings than obtained by VHE. Subsequent measurement by VHE showed that hydrogen was retained in the steel using the encapsulation method. The most likely explanation for this is that, during hydrogen evolution at 400°C in the capsule, a positive hydrogen partial pressure is developed, hindering further liberation from the weld deposit: this is consistent with the apparent complete evolution in the sample encapsulated with a titanium getter. This observation has also been made by other workers ^[16] . On the other hand, Kaçar ^[17], performed repeat analyses on duplex weld deposits using the encapsulation technique, and did not report any further hydrogen evolution on reheating. Clearly, further work is required in this area before the generally more convenient encapsulation/gas chromatography approach can be accepted for routine hydrogen analysis.

4.2 Hydrogen Cracking Tests

4.2.1 General Comments

The programme sought to generate cracking data for SMA welds, in 22%Cr duplex stainless steel, for which the consumable hydrogen level had been determined using the VHE method. Cracking occurred in welds containing ~83% ferrite and hydrogen levels of 11.4 and 17.5ml/H 2

4.2.2 Comparison With Other Work

Lundin et al carried out Y-groove cracking tests using both duplex and superduplex stainless steels, with SMA weld deposits [4,5]

% ferrite = 0.57FN + 8.82 (for 22Cr duplex stainless steel, ^[20];
% ferrite = 0.82FN + 3.6 (for 25Cr superduplex stainless steel, ^[21] .

The current work agrees well with the results of Lundin et al, accepting that the reported hydrogen levels of the welds are probably lower than actual.

Walker and Gooch ^[5] used a similar experimental approach to the present investigation, and carried out Y-groove cracking tests on 22%Cr duplex stainless steel with commercial SMA consumables. The deposited metal hydrogen content was determined by VHE. Welds containing ~60% ferrite and 12-16ml/H ₂ /STP/100g deposited metal did not crack. These results compare favourably with the current work.

It is possible to include in Fig.5 a boundary curve, separating cracking and no cracking conditions for SMA deposits in duplex stainless steel. It should be noted that there exists some degree of uncertainty as to the shape of the curve in Fig.5 at ferrite volume fractions between 65 and 85% because of a lack of data, and further study is desirable.

4.2.3 Practical Implications

5. Conclusions

1. The VHE method of hydrogen analysis is applicable to duplex stainless steel weld metals. Table 11 presents guidance as to the minimum time required to extract hydrogen using VHE equipment in the 400 to 950°C temperature range.
   
   
2. Fabrication hydrogen cracking in SMA 22%Cr duplex stainless steel weld deposits may be avoided by maintaining a weld metal phase balance of less than 60% ferrite and a deposit hydrogen content of less than 18ml/H ₂ /STP/100g.
   
   
3. A predictive diagram has been generated, using the results of the current work and other published data, to aid the prevention of fabrication hydrogen cracking in practice.

6. Acknowledgements

7. References

Copyright by TWI, 1999