Best Welding Practices for Heat-Resistant Alloys
Heat-resistant alloy fabrications are used extensively in the thermal-processing industry for items including fixturing, retorts, radiant tubes and more.
In the U.S., these fabrications are constructed primarily from austenitic, nickel-based alloys. It is common for welders, whose primary experience is steel welding, to weld these alloys and encounter cracking. The high-nickel and chromium contents make these alloys austenitic and not subject to any phase transformation. Therefore, the considerations for obtaining a good, sound weld are quite different for these austenitic alloys. By understanding that there are differences between carbon steels and nickel-based heat-resistant alloys and employing proper procedures and practices, these alloys can be successfully welded consistently.
For the purposes of this article, Figure 1 shows typical wrought nickel-based heat-resistant alloys and their nominal chemistries. The list is not exhaustive.
Chemistry and Metallurgical Considerations
Nickel-alloy welds will stay austenitic during solidification; there is no martensitic transformation. Therefore, in order to successfully weld, the tendency of the weld to hot tear must be overcome. To accomplish this, weld fillers with the lowest possible residuals are essential. At a minimum, these nickel alloys must be joined with weld fillers of matching composition. Over-alloyed weld fillers are often needed to impart the required corrosion resistance and strength for the intended service.
Carbon steels are increasingly likely to crack as carbon content increases, especially when they contain more than 0.25% C. As more heavily alloyed steels cool after being heated into the austenitic range, they form a nonequilibrium phase called martensite, which is susceptible to cracking if not tempered. In a process such as welding, the moisture in the air can be enough for hydrogen pickup and subsequent cracking. The martensitic phase is particularly susceptible to underbead hydrogen cracking. Therefore, preheating and post-heating is important in steel welding. These two processes are used to slow down the rate of cooling enough so that martensite formation is either minimized or eliminated.
The austenitic nickel-based alloys do not harden no matter how fast they are cooled because they are not subject to any phase transformations. Instead, high-nickel alloys are susceptible to cracking in restrained joints, heavy sections or slow-cooled welds. The weld bead tears rather than stretches when the bead contracts during solidification. This is called hot tearing. The faster a nickel-alloy weld freezes, the less time it spends in the temperature range where it can tear. For successful welding of nickel alloys, it is important to quickly extract the heat.
Given these considerations, there are multiple parameters that require control for successful welding of nickel-base alloys including: best welding practices, surface preparation, proper shielding gas, proper penetration and low heat inputs.
Good Welding Practices
Much of the cracking observed at manufacturing and fabrication facilities is the result of the welder inadvertently increasing the heat input to the weld. The most common way that this occurs is by oscillating the weld bead from side to side as the weld is being made. This practice tends to slow the weld speed down and, therefore, allows more heat to be put into the material. The typical telltale sign of this is the weld crown shape. Instead of showing a nice convex crown, the weld is either flat or sinks down to concave. When this happens, the centerline of the weld is weakened, and a crack can occasionally be seen running down the weld centerline. This is shown in Figure 2.
The second problem seen is an uneven weld. In such cases, the width of the weld varies as the speed varies. The width is not uniform along the weld profile. In such welds, one can see peaks and valleys as well as wide and less-wide variations. This situation may also lead to cracking.
As stated previously, the heat input must be kept low. When multiple-pass welding is required, the metal should be allowed to cool to less than 212°F (100°C) before attempting the next pass. This should be physically measured with a device such as a probe or thermocouple directly on the weld crown.
Make full-penetration weld joints. Incomplete welds that form a channel between two pieces must be avoided. Incompletely penetrated welds will not provide the required strength for many applications. A good analogy would be a tensile test of different sizes of the same material. The strength would be the same in psi. Since stress is force per unit area, however, it will take more applied force to yield and break the thicker sample than the thin. Incomplete-penetration welds behave just as the thinner metal in the analogy behaves. Less force will be required to break it. Furthermore, the unwelded region concentrates stress so that the joint fails by thermal fatigue. The crack grows from that unwelded zone inside the joint, through the weld bead to the outside. This is depicted in Figure 3.
In order to obtain good penetration, grooves and bevels can be machined into one or both workpieces (Fig. 4).
One other observation seen by welders is that cracks can develop from the very end of the weld. These are called crater cracks. This can be remedied by backstepping the weld approximately ¼ to ½ inch from the end of the weld before the weld solidifies. Ideally, this is done as soon as the welding pass is complete (Fig. 5).
All welds must be done on prepared edges that are clean and free of all high-temperature scale. These austenitic nickel-based alloys are usually furnished in the annealed and pickled condition, which is acceptable. In the rare instances where they are supplied with the mill scale intact, the scale must be ground off. Failure to do so will result in poor adhesion.
All grease, oil and paint, especially zinc paint, must be thoroughly cleaned and removed from the surfaces prior to welding. Sulfur-bearing oils can lead to cracking in alloys with high-nickel contents. The carbon in grease and oil can result in a slight boost of carbon content, which will diminish corrosion resistance in the area where the grease and oil was in direct contact with the metal.
When multipass welds will be made and a flux process is used, it is imperative to physically remove all traces of flux before starting the next pass. This slag, or flux, is extremely corrosive at high temperatures. Slag pieces left on the weld bead can destroy the protective chromium-oxide layer on the metal as the next pass is deposited. Under oxidizing conditions, there will be excessive loss of metal due to oxidation. Under carburizing conditions, there will be rapid localized carburization.
A commonly asked question is what shielding gas should be used for welding heat-resistant alloys. The simple answer is that it depends on the welding process used and, in the case of MIG welding, the method of material transfer. The right gas is also dependent on the grade of weld wire being used.
In general, argon is the primary shielding gas for TIG or GTAW welding. For RA 602 CA, it is essential to have a small addition of nitrogen to prevent cracking (2% is essential and sufficient). Higher quantities of nitrogen can erode the tungsten electrode.
MIG, or GMAW, is unique in that there are three different methods of material transfer: spray arc, pulsed arc and short-circuit arc. In general, the gases required for spray-arc and pulsed-arc transfer are the same. They are typically argon with varying amounts of helium and sometimes small amounts of carbon dioxide. In the case of RA 602 CA, a more complex blend of argon with 10% helium, 5% nitrogen and traces (0.05%) of carbon dioxide is required. When the alloy was first developed, Linde supplied a trademarked gas blend called Cronigon N30. This blend now goes by the trade name Z-ArHeNC.
Many alloys will need to be root shielded. Some of the reasons include keeping a key alloying element in solution instead of oxidizing out or preventing reactions between an air atmosphere and one or more alloying elements. Once the first pass is laid down, there is no further need for root shielding. However, every pass must be shielded.
In stick welding, or SMAW, some shielding gas is in the flux coating. Between these gases and the flux itself, there is sufficient protection. SMAW should not be used on alloys where root passes are to be made.
In order to ensure a good crack-free weld, many will run die-penetrant inspections to check for cracks, or lack thereof. Some additional suggestions include:
- Check the weld contour. The entire weld should be convex with a nice gradual crown. If the crown has a steep, hilly contour, there could be long-term problems.
- Check the width of the weld. It should be consistent and uniform. If there are hourglass-shaped variations, the weld quality is subpar and could cause a premature failure. Oftentimes, this is an indication of penetration problems.
- Does the weld show complete penetration? Lack of full penetration allows gas buildup and corrosion to start in the channel where the full penetration is lacking.
For more information: Contact Marc Glasser at Rolled Alloys, 125 West Sterns Road, Temperance, MI; tel: 800-521-0332, e-mail: firstname.lastname@example.org; web: www.rolledalloys.com. Various welding and fabrication manuals found on www.rolledalloys.com/technical-resources/ and from James Kelly’s Heat Resistant Alloys were utilized. 602 CA® is a registered trademark of VDM Metals.