High-strength steels in pipe
For transmission and process pipe fabrication, some applications require elevated strength and toughness levels. In these cases, high-strength steel may be the right option.
High-strength steels are being used more frequently across many industries and welding applications, including pipe fabrication and construction, that require higher strength properties without added weight. While high-strength steels deliver numerous benefits, welding these materials requires more attention to certain details, including heat input and filler metal selection.
Learn more about high-strength steels and get tips for producing the best results when welding them in pipe applications.
What is high-strength steel?
The term high-strength steel refers to base materials and filler metals that have elevated tensile and yield strengths. Yield strength is the amount of force that can be imposed on a material that will still allow the material to revert to its original form once that force is removed. This is called elastic deformation. When force greater than the yield strength is applied and the material doesn’t return to its original shape, it’s called plastic deformation. Ultimate tensile strength is the amount of applied force needed to cause fracturing. Because high-strength steels have higher tensile and yield strengths, they don’t flex as much under heavy loads, resulting in a lower chance of deformation or breakage compared to mild steel.
And because these materials have an elevated strength-to-weight ratio, they provide these strength properties without added material thickness or weight compared to mild steels.
The result? Contractors can use thinner, lighter materials for pipes that must withstand higher pressure. This means less steel needed in the fabrication process, reduced transportation costs for moving or shipping, and less time spent welding due to the thinner walls.
Common grades of high-strength steel include X52, X65, X70 and X80. The larger the grade number, the higher the yield strength. The added strength comes from alloying elements. In the past, carbon was often the alloying element used to produce high-strength steel for pipe applications. However, many of today’s high-strength steels use more manganese, nickel, niobium, vanadium, chromium or titanium as alloying elements. This minimizes the carbon content, helping to reduce the potential for hydrogen-induced cracking by potentially lowering the carbon equivalent.
Even so, it’s important to carefully control the heat input and choose the right filler metal for the material and application to get the best results when welding high-strength steels.
Pay attention to heat input
Because high-strength steels can be more prone to cracking than mild steels, utilizing proper preheating and interpass temperatures is key. Preheat helps slow the rate of cooling to prevent the formation of a brittle microstructure within the weld and heat-affected zone (HAZ) that can result in hydrogen-induced cracking. It also helps reduce the residual stress that can build up during welding.
The necessary time and temperature for proper preheating vary since they are dependent on material type and thickness. The required preheat temperatures are typically dependent on the procedure and code being followed. Material types X60, X70, X80, etc., and thickness will vary in preheat temps from 50 to 250 degrees Fahrenheit, but keep in mind that the code and procedure will dictate the temperature.
Be sure to monitor preheat temperatures to make sure they stay within the required range for the material thickness, carbon equivalent range and desired mechanical properties for the finished weld.
Using preheat temperatures that are too low can lead to higher hardening in the HAZ, causing it to become brittle. On the flip side, preheat temperatures that are too high can soften the material and result in lower tensile strengths and reduced toughness by causing large grain growth. Both scenarios can potentially result in weld or HAZ failure.
The balancing act of finding the proper preheat window should be addressed during procedure qualification and spelled out in the welding procedures.
Using induction heating technology for preheating, interpass heating and post-weld heat treat can provide numerous benefits when compared to other heating processes. Induction offers a fast time-to-temperature, provides consistent heating and delivers safety benefits for operators. In addition, newer technologies allow for more portable induction heating solutions designed for jobsites that can be powered by engine-driven welder/generators.