Super duplex steels are often specified for oil and gas, water and desalination industries for their corrosion, tensile and low temperature impact properties. However, these steels can suffer from the formation of deleterious phases which adversely effect both corrosion and impact properties.
With 6A-G material, sigma phase precipitation can be eliminated or significantly reduced leading to dramatic improvements in both low temperature impact properties and pitting corrosion resistance, with no reduction in through section tensile strength.
This extends significantly the section size that can be produced. Additionally, previous operational temperature constraints are substantially improved as 6A-G can operate not only in Arctic conditions with excellent impacts at -76°C, but also in seawater corrosion applications above 50°C as high as 60°C.
There is a step change in the improvements of mechanical and metallurgical properties achieved in the parent cast and weld metal when very specific chemistry and process parameters are adhered to during manufacture.
This development work has resulted in the reduction of the section size limitations of SDSS castings, forgings and welds, and the vast improvement in corrosion and impact properties of both heavy and thin section castings and forgings
| G48 Method ‘A’ Pitting Corrosion | ||
|---|---|---|
| Material 50mm Section Tested at ½T | Conventional Test Temperature | New Test Temperature |
| Parent | 50° | 60° |
| Weld 'AS Welded' | 40° | 50° |
| Parent | 50° | 60° |
| Acceptance | No pitting; weight loss max 4g/m2 | |
The 6A-G material has improved pitting resistance due to its enhanced chemistry, cleanliness, and reduction of intermetallic phase precipitation. Two standard corrosion tests are used in the routine production testing of SDSS, ASTM G48 Method A and ASTM A923 method C, the former being the more commonly stipulated.
Both tests use ferric chloride mixed with water (10% Fe3Cl•6H2O) as the corrosion media, and for parent material SDSS the G48 test temperature normally stipulated is 50°C with a duration of 24hrs.
Fig. 3 shows the comparison between the weight loss of 6A-G SSDS compared with the weight loss of conventional 6A SDSS cast material during G48 method A testing at 60°C.
This temperature was selected to test the material beyond the standard test regime of 50°C to demonstrate the enhanced resistance capable with the 6A-G material.
The industry standard maximum allowable weight loss is 4g/m2, figure 3 demonstrates that all 6A-G heats have a weight loss less than the maximum allowable for 50°C, but is achieving this at 60°C. The conventional 6A cast material heats all fail to pass the weight loss restriction by a large margin.
6A-G has enhanced hydrogen induced stress cracking resistance (HISC). For those who are more familiar with this phenomena it will be no surprise that resistance to HISC is improved in the 6A-G material due to significantly superior corrosion resistance at higher temperatures.
6A-G SUPER DUPLEX WELD METAL
Goodwin research and development on SDSS weld metal originally focussed on deep welds in SDSS castings.
Conventional weld qualifications for ASTM weld materials are governed by ASTM A488 where the impact test location is stipulated just below the weld cap. However, Goodwin discovered that after subsequent post-weld heat treatment impact results at depths greater than 35mm dramatically reduced to unacceptable values using conventional commercial fillers, while the parent material was unaffected.
Commercial fillers for 6A super duplex parent material are generally split into two groups, parent matching or over alloyed. During the testing regime, both types of consumable were tested. Upon investigation on multiple thick section tests it was demonstrated that the conventional weld metal responded differently to the parent material during post weld heat treatment and began to precipitate sigma (σ) phase much more quickly than the parent material during quenching from post heat treatment temperatures.
This presented much less of a problem if the weld could be left in the 'As Welded' condition, but where specifications called for mandatory post weld heat treatment (PWHT) this created at the time an insurmountable problem with current commercially available filler metals.
Goodwin worked in conjunction with WB Alloys to develop a filler material that has a quench response with regards to sigma (σ) phase precipitation kinetics much closer to that of the parent material.
Fig 4 shows the marked improvement in 'As Welded' properties using the 6A-G filler metal compared with standard parent matching and over alloyed with nickel consumables.
Fig 5 shows one of the major findings of this work, which is that conventional fillers have a significant drop off in impact properties when welded in thick sections and subsequently post weld heat treated. PWHT is mandated by ASTM A995 for duplex castings with welds of greater depth than 25mm and as a supplementary requirement for the same depth of weld repair for duplex castings to ASTM A890.
SDSS 6A-G Impact Properties in the 'As Welded' condition;
For conventional fillers at 100mm depth impact values are in single figures as a function of the precipitation of sigma phase in the PWHT condition.
The new 6A-G filler metal also shows a reduction in impact properties as the weld depth increases. However, even at the 100mm depth the impact properties at -46°C are over 1.5x the minimum 45J required by most weld qualification requirements, and at 25mm depth the results are over 4x the value of the conventional filler in the PWHT condition.
This is achieved whilst controlling the interpass temperature to 150°C and with deposition rates more than double current welding rates if the TIP-TIG process is utilised.
SDSS 6A-G Impact Properties in the PWHT condition; (Solution Treated + WQ)
Figure 6 shows results of G48 method ‘A’ pitting corrosion testing for 24hr duration at a variety of temperatures. G48 method A is specified in the oil industry to validate weld qualification procedural qualifications.
In the ‘As Welded’ condition the test temperature is specified at 40°Cfor a duration of 24hrs with a final maximum weight loss of >4g/m2 with no visible pitting allowed.
Now with 6A-G filler in the ‘As Welded’ condition, this can pass G48 Method 'A' at 50°C with pass data up to 60°C rather than 40°C. In the PWHT condition it can pass at 60°C with pass data up to 65°C where this was previously limited to 50°C
SDSS 6A-G 20mm Thick Butt Weld; G48 Method 'A'Corrosion Results; Duration 24 hours
