Q: Why are some suppliers reluctant to produce duplex stainless-steel valves that are compliant with NACE MR0103?
A: Duplex stainless steels are those that are chemically balanced to provide a target microstructure of 50% austenite phase and 50% ferrite phase. Duplex stainless-steel valves required to meet NACE MR0103 will almost always be installed in a sour, corrosive, chloride-containing environment, so it is essential the material provides optimized resistance to sulfide stress cracking, chloride stress corrosion cracking, chloride pitting and general corrosion.
The task group that created the NACE MR0103 document, “Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments,” recognized that microstructure is a key factor influencing the properties of duplex stainless steel. They also recognized that the microstructure is influenced strongly by chemistry and by heating and cooling during processing of the material. As a result, NACE MR0103 includes some very stringent requirements for duplex stainless steels, especially with regard to welding. Since most valve bodies (and many bonnets) are produced from castings, welding will be performed. Although most welding will be minor and cosmetic in nature, NACE MR0103 does not distinguish between minor and major weld repairs when it comes to welding requirements for these materials.
During welding, the cooling rates that occur in the weld deposit and in the heat-affected zone (HAZ) can have dramatic effects on the final microstructure. Higher cooling rates tend to promote higher ferrite levels, which are advantageous for chloride stress corrosion cracking resistance but detrimental to sulfide stress cracking resistance and impact resistance. Conversely, slower cooling rates promote lower ferrite levels, which will result in increased sulfide stress cracking resistance and toughness, but reduced chloride stress corrosion cracking resistance, and can allow the formation of harmful second phases that detract from the overall corrosion resistance of the material.
Therefore, successful welding of duplex stainless steels to provide optimum properties is a fine balancing act, which requires careful control of welding parameters that affect the cooling rate of the weldment.
In order to achieve the necessary control over cooling rates during welding, NACE MR0103 includes several requirements with regard to qualification of welding procedures:
- Each procedure qualification specimen must be hardness surveyed using to 10 kg Vickers hardness scale. The readings must average no more that 310 HV 10, and no individual reading is allowed to exceed 320 HV 10.
- The ferrite content must be measured metallographically in the weld deposit and HAZ of each procedure qualification specimen. The measured ferrite content must fall within the 35% to 65% range in all measured locations.
- The heat input required per the resulting welding procedure specification must not deviate by more than 10% from the heat input used when producing the procedure qualification specimen(s).
- The base metal thickness range qualified by a given procedure specimen of thickness T is 0.8T to 1.2T.
Requirements 1 and 2, although they add testing over and above the normal ASME Section IX requirements, are not overly problematic.
Requirements 3 and 4 are not so easy. Heat input is calculated by multiplying voltage x current x 60 and dividing by travel speed. During welding, voltages and currents vary depending upon a number of factors, including the distance from the electrode to the work piece. Travel speed can also vary, especially in manual welding processes. In producing weld repairs, travel speed is very difficult to control, even if repairs are produced with stringer beads. Many in the industry believe the only way to maintain a level of heat input that does not deviate by more than ±10% is to employ automated welding methods, which would not be useful for repair welds.
The qualified thickness restrictions are also problematic from a cost standpoint. For example, assume a valve company produces a wide range of valve sizes and pressure classes. In order to be able to produce valve bodies ranging from 1-inch through 10-inch nominal size, in ASME pressure classes ranging from 150 through 2500, the company would have to be prepared to weld repair on wall thickness ranging from 0.25 inch (1-inch Class 150 minimum body wall) through at least 7.875 inches (10-inch Class 2500 flange thickness). This would require nine PQR specimens of graduated thickness starting at 0.313 inches thick, and increasing by 50% each step. The final coupon would need to be at least 6.3 inches thick. The costs to procure all of the raw material, machine the coupons, weld the coupons and perform all the necessary testing are very high. There also is no guarantee all of the coupons will pass all of the tests on the first attempt.
Add to this the fact that most projects calling for NACE MR0103-compliant duplex also carry extra requirements such as low-temperature impact testing and special corrosion tests, and the costs and opportunity for failure of qualification tests increases even more.
The appropriate welding procedures would need to be developed not only at the sites where the castings are machined (in case they run across any casting defects during machining), but also at each foundry that would be supplying castings.
Considering all of these issues, it is difficult to justify proactively developing the welding procedures when considering the low number of requests for duplex stainless-steel valves for refining applications, especially where NACE MR0103 compliance is also required. Without appropriate welding procedures, it is virtually impossible to quote NACE MR0103-compliant duplex stainless-steel valves.