June 17, 2008
When welding a chromium-molybdenum alloy, selecting the optimal filler wire is critical to the long-term durability of the weld. Fortunately, matching the filler metal to the alloy is no more difficult than it is for matching a filler metal to any other family of alloys. Understanding the chemical and mechanical properties of the materials can go a long way in making strong, corrosion- and creep-resistant welds.
Chrome-moly pipe has become a standard in industries such as power generation, chemical processing, and petroleum refinement, not only for its corrosion resistance and high-temperature strength, but also for its cost-effectiveness. In many applications, it is a viable alternative to a more costly stainless steel pipe.
Choosing filler metals to weld chromium-molybdenum pipe requires the same principal consideration as for any material: Match the chemical and mechanical properties in a way that yields the strongest, safest, and longest-lasting welds. Specific low-alloy shielded metal arc welding (SMAW) electrodes and flux-cored arc welding (FCAW) wires do just that—they provide the low-carbon, low-hydrogen properties that prevent welds from corroding or cracking and the strength to withstand high pressures and temperatures. It just takes a little know-how to choose which is the best one for the job.
To select low-alloy filler metals, it is important first to understand chrome-moly pipe itself. Among the more common grades are P11, P21, P22, P91, and P92, some of which can withstand service temperatures higher than 1,112 degrees F (600 degrees C) and have wall thicknesses from 1⁄8 inch to 8 in. The amount of chromium, molybdenum, and other alloying elements determines the assigned grade. These particular grades contain 11⁄4 percent to 9 percent chrome and 1⁄2 percent to 1 percent molybdenum. As the chrome content increases, so does the corrosion and temperature resistance. Not surprisingly, the amount of chrome is also a determining factor in filler metal selection.
When welding chrome-moly pipe, regardless of the material grade or the filler metal being used, you must maintain specific preheat and interpass temperatures. Consistent temperatures help preserve strength and crack resistance under extreme service conditions. Recommended preheat temperatures are from 250 to 400 F (121 to 204 C) according to the wall thickness (see Figure 1).
Per AWS D10.8-96, "Recommended Practices for Welding of Chromium-Molybdenum Steel Piping and Tubing," you must preheat the pipe, even before tack welding. It is especially important to preheat before repairing P21 and higher grades that have been in service because the pipe may have experienced temper embrittlement or ductility loss. This can occur when the pipe has been exposed to temperatures above 850 F (450 C) for an extended period of time (ref. AWS D10.8-96, 5.1 and 7.2).
Maintaining interpass temperatures is also important. A range of 350 to 600 F (177 to 316 C) is typical. The specific temperature range depends on the grade of chrome-moly and its thickness, but most importantly on the required welding procedure.
Postweld heat treatment (PWHT) or stress relieving is also recommended. PWHT requirements are generally 1,150 to 1,400 F (621 to 760 C) for one hour. PWHT helps rid the weld of hydrogen that may have been picked up from the filler metal, the base metal, or the atmosphere and can help minimize the chances of cracking. Most low-alloy filler metals designed for chrome-moly applications also come with recommended stress-relieving temperatures and durations, either on the package label or on the included specification sheet.
Low-alloy SMAW electrodes are the choice for chrome-moly pipe welding repairs, because they are easily portable (they do not require shielding gas).
They can also reach into the small physical confines of existing chrome-moly piping systems.
As a rule, low-alloy stick electrodes designed for welding chrome-moly have low hydrogen levels and are characterized by low spatter, fast-freezing and easy-to-remove slag, and good bead wash and tie-in, all of which are intended to make welding repairs fast and trouble-free. To maximize the benefits of these characteristics, clean the pipe before welding. Often chrome-moly picks up hydrogen during normal service, which can lead to cracking in new welds.
For P11 chrome-moly pipe, use either an AWS E8018-B2 H4R or E8018-B2L H4R SMAW electrode; both are formulated for applications subject to high heat and humidity (see Figure 2). They also resist hydrogen pickup that can lead to cracking or starting porosity. The E8018-B2L H4R stick electrode contains a lower amount of carbon, which further protects against cracking. Both electrodes work especially well on boiler and similar piping repairs that require weld tensile strengths higher than 80,000 pounds per square inch (PSI). On average, they offer tensile strengths from 90,000 to 98,000 PSI.
Pipe alloys P21 and P22 require SMAW electrodes with higher chrome content and higher tensile strengths than those used for P11 pipe. Options include AWS E8018-B3L, E9018-B3 H4R, or E9018-B3L H4R, which are good for boiler work and general piping applications. These SMAW electrodes yield tensile strengths from 105,000 to 110,000 PSI and generally feature a special moisture-resistant coating that protects against hydrogen pickup in hot, humid service conditions.
Finally, when repairing P91 or P92 pipe, choose a SMAW electrode formulated to improve creep resistance. Creep is any deformity (such as buckling or warping) caused by stress or heat during the welding process, and higher-strength chrome-moly is prone to this phenomenon. AWS E8018-B8 H4R, E8018-B8L H4R, or E9018-B9 SMAW electrodes can help provide that protection and are good choices for petrochemical and petroleum pipes and other surface applications with service temperatures that exceed 300 F (149 C). They have tensile strengths between 100,000 and 110,000 PSI.
As with any SMAW electrode, proper storage is key to preventing moisture pickup. Dry, cool areas are ideal. If reconditioning is necessary, follow the filler metal manufacturer's recommendation for temperature and duration.
Low-alloy flux-cored wires are generally reserved for fabricating chrome-moly pipe, as opposed to repairing it. Two primary reasons for this relate to welding speed and portability. First, FCAW is faster in preassembly work, where productivity is key. Second, most low-alloy flux-cored wires are gas-shielded, which makes them much less portable for repairs (they require an external shielding gas tank). Low-alloy flux-cored wires can be used for repairs, however, if the repairs are in an area with sufficient access and the necessary equipment is available.
Like low-alloy SMAW electrodes, low-alloy FCAW wires are designed with fast-freezing, easy-to-remove slag and low hydrogen levels. They also offer high deposition rates and good weld penetration.
The following flux-cored wires are suitable when fabricating P11 chrome-moly pipe: AWS E80T1-B2C/M, E80T5-B2C/M, and E81T1-B2C/M, all of which offer excellent strength and creep and corrosion resistance in high-service-temperature applications. Their characteristics are comparable to an E8018-B2 SMAW electrode's, but yield different tensile strengths according to the shielding gas used. A letter C at the end of the AWS classification indicates the wire requires 100 percent CO2, whereas an M indicates the wire uses a 75 percent argon/25 percent CO2 mixture and provides higher tensile strengths that wires that use straight CO2. The argon increases the deposition rate; increasing the amount of metal flowing into the weld increases its strength.
All of the wires are designed for single- or multipass welding. However, the E80T1-B2C/M and E80T5-B2C/M can be used only in the flat and horizontal positions, which means the pipe must be rotated during welding. These flux-cored wires offer a tensile strength of approximately 99,000 PSI.
The E81T1-B2C wire can be used in all welding positions. It requires 100 percent CO2 shielding gas and creates welds with around 96,000-PSI tensile strength, whereas the E81T1-B2M uses a 75 percent argon/25 percent CO2mixture and can yield tensile strengths greater than 100,000 PSI. Regardless of the shielding gas or the strength, this type of wire helps resist hydrogen pickup when the pipes enter into the service environment.
The main low-alloy flux-cored wires for welding P21 and P22 chrome-moly pipe include AWS E90T1-B3C/M and E90T5-B3C/M for flat and horizontal welding, and the E91T1-B3C/M H4 for all-position welding applications. The E91T1-B3C H4 requires 100 percent CO2, and the E91T1-B3M H4 uses 75 percent argon/25 percent CO2 shielding gas. Like other low-alloy flux-cored wires, these offer good corrosion and cracking resistance. They are good choices for fabricating pressure piping and steam or chemical pipes requiring strengths more than 100,000 PSI.
Typically, the best low-alloy flux-cored wire for welding P91 and P92 chrome-moly is an AWS E91T1-B9. This type of wire offers a fast-freezing slag for out-of-position welding, along with low diffusible hydrogen levels to control cracking. It also has a very high tensile strength: approximately 126,000 PSI. Not surprisingly, because P91 and P92 chrome-moly pipe are used for higher temperature services than the other grades, E91T1-B9 flux-cored wire creates X-ray-quality welds, which are essential on such critical piping applications.
TPJ - The Tube & Pipe Journal® became the first magazine dedicated to serving the metal tube and pipe industry in 1990. Today, it remains the only North American publication devoted to this industry and it has become the most trusted source of information for tube and pipe professionals. Subscriptions are free to qualified tube and pipe professionals in North America.