Increasingly demanding material requirements found in piping and boiler construction for industries such as power generation and petrochemical have made a specific type of chromium-molybdenum steel pipe — referred to as grade P91 — a frequent choice because of its ability to withstand high pressures and high temperatures. P91 is used in many critical applications that require significant strength at high temperatures, including superheaters, headers and steam lines.
However, P91 is a higher-cost material that presents some challenges in fabrication. Achieving sound welds with P91 pipe requires operator skill; careful control of heat treatment before, during and after welding; and choosing appropriate low-hydrogen filler metals.
In addition to following specific procedures and best practices to achieve high-quality welds, a change in welding processes can lead to significant productivity gains and reduce the time it takes to train new welding operators in P91 welding applications.
While many contractors have used traditional processes of TIG and stick to weld P91 pipe, converting to wire processes like modified short-circuit MIG, pulsed MIG and flux-cored welding in P91 fabrication can save significant time — reducing welding time by half on some jobs. This is due to several factors, including greatly increased travel speeds and deposition rates, processes that are easy to learn and use, and the ability to eliminate the back purge in some cases. This helps contractors improve profitability and be more competitive, without sacrificing the level of quality necessary for this critical work.
The basics of P91
P91 is a type of creep strength enhanced ferritic (CSEF) alloy, which are steels designed to retain strength at extremely high temperatures. The P91 abbreviation represents the material’s chemical composition: 9 percent chrome and 1 percent molybdenum.
The material is used in high temperature, high pressure steam piping not only because it retains strength at elevated temperature but also because it resists failure due to “creep,” which is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses. P91 also resists corrosion better than steel alloys used previously in these applications. Because of these advantages, P91 is used extensively in power generation applications, such as for high pressure steam lines.
Due to the critical nature of these applications, any defects in the weld can result in cracking and eventual part failure. Failures in high-pressure steam pipe can be catastrophic, causing ruptures, tank or valve explosions, or other serious incidents that can result in fatal injuries. As a result, codes and procedures for P91 applications are stringent.
Control the heat
While CSEF alloys such as P91 are designed to maintain strength as the environmental temperature rises, one of the biggest challenges in welding P91 is the material’s sensitivity to heat changes during the welding process. It’s critical to properly control heat input — before, during and after welding. Too much or too little heat can lead to issues with cracking in the heat affected zone (HAZ) and subsequent premature failure of the weld.
The HAZ represents a region of potential weakness in P91 welding — and a major factor in the quality and service life of the weld. How quickly the material cools and the weld puddle solidifies are factors that influence the material’s microstructure.
Preheating helps drive off moisture and reduce hydrogen, and it also reduces the thermal gradient between the base material and the weld puddle to improve weldability and keep the weld pool from cooling too quickly. Typically, P91 must be preheated in a temperature range from 400 to 600 degrees Fahrenheit, though the appropriate heating level depends on the qualified weld procedure used for construction and fabrication of the piping system.
During the welding process, it’s equally important to keep the interpass temperature range within the approved qualified weld procedure. Too cool and cracking may occur; too hot and the material can lose its strength and toughness. In addition, keep in mind that the more heat applied and the longer heat is applied, the larger the HAZ, which can increase opportunities for cracking.
Another critical step is post-weld heat treatment, which returns the material’s microstructure to a more favorable condition. The heat of the welding process makes the weld deposit and neighboring HAZ brittle when welding P91. Post-weld heat treatment restores toughness to the weld deposit and the HAZ.
Because precision and stability in temperature control are extremely important when welding P91 pipe, induction heating is a solution that offers better control and more uniform heating of the part. Induction, a form of electric preheat, does not rely on a heating element or flame to transfer heat. Instead, an alternating current passes through the device, creating a magnetic field around it. As the magnetic field passes through the conductive workpiece, it creates eddy currents within the part. Resistance to changing electrical eddy currents generates heat in the part. The part becomes its own heating element, heating from within, which makes induction very efficient since little heat is lost in the process.
Because induction maintains uniform heating through the heat zone, it reduces the potential for hot and cold spots in the part. Induction systems also allow welding operators to check and adjust the temperature as needed — and document preheat, interpass and post-weld temperature levels for quality assurance, code requirements or customer specifications.
Consider advanced wire processes
Another factor that can contribute to P91 weld quality — and significantly impact productivity and efficiency for contractors — is the welding process used. Advanced wire processes such as modified short-circuit MIG and pulsed MIG offer numerous benefits and can help contractors improve profitability and be more competitive.
With modified short-circuit MIG, travel speeds are three to four times those of TIG or stick welding. In addition, using a modified short-circuit MIG process allows for the elimination of a back purge in some cases, thanks to the consumables used and the lower heat input. Typically, when P91 pipe is TIG-welded, it requires an argon back purge during the root pass and for some time afterward. Eliminating this saves in wasted gas and the time it takes for the purge. Also, with advanced waveform MIG the thicker root deposit helps eliminate the hot pass, therefore reducing the risk of burnthrough on the subsequent fill passes. This also helps eliminate the need for a back purge.
A modified short-circuit MIG process anticipates and controls each short circuit, then reduces the available welding current to create a consistent metal transfer. This precisely controlled metal transfer provides uniform droplets, creating only small ripples in the weld puddle and producing a consistent tie-in to the sidewall. Along with its ability to maintain the same arc length regardless of stick-out (within limits), this process makes it much easier for welding operators to control and manipulate the puddle, and to quickly and easily learn to create uniform, high-quality welds.
While a process switch may require an investment in operator training, converting to a process that offers ease of use and greater productivity can help contractors address the labor shortage facing the industry. An easier-to-learn MIG process can shorten training time, so welding operators can be on the job making quality welds sooner.
The mindset regarding traditional processes may be “why fix what’s not broken,” but it’s becoming increasingly clear that changing industry pressures — including the skilled labor shortage, shorter project timelines and a greater focus on controlling costs — are causing traditional fabrication methods to become “broken” from a profitability and efficiency standpoint.
Simplifying the process with a switch to modified short-circuit MIG for the root pass followed by pulsed MIG or flux-cored welding for the fill and cap passes can help contractors control costs and save time during welding to accelerate project completion. An investment in these processes and training can pay off quickly — without impacting weld quality.
The filler metal X-factor
Another key to success when welding P91 is choosing the right filler metal for the application. The performance of P91 welds depends on having the correct chemical composition in the weld metal; therefore, it is highly recommended that filler metals be purchased with test reports showing actual chemical analysis for the specific heat/lot combination that one has purchased.
Also, controlling hydrogen in the weld to avoid hydrogen-induced cracking is important with any high-strength alloy, and P91 is no exception. Look for low-hydrogen filler metals designed for use with high-strength chromoly materials.
Datasheets for all chromium-molybdenum filler metals should provide a typical X-factor designation for the product. This formula measures a weldment’s resistance to temper embrittlement. The number is calculated from the amounts of four key contaminants (elements) in steel: phosphorous, antimony, tin and arsenic, which together have the greatest impact on a weldment’s susceptibility to temper embrittlement. Here is the formula: X = (10P + 5Sb + 4Sn + As)/100.
It’s especially important to know the X-factor of a filler metal when welding certain chromoly steels, such as P91. Look for a filler metal with an X-factor below 15 for P91 welding applications.
Success with P91
Quality is critical in the demanding, high-pressure applications that utilize P91 pipe. Contractors must meet these stringent quality standards, while also addressing the increasing expectation that projects be completed faster — even as the welding industry faces a shortage of skilled labor.
Carefully controlling heat input and choosing the right filler metals is important when welding P91 pipe. In addition, a change in welding processes can help contractors save time and money and gain the competitive advantage they seek.
While the risks and rewards should be considered for each individual project, a change in welding processes can help contractors achieve significant productivity gains to improve profitability, without impacting the high quality necessary for P91 welding.