How To Optimize Results When Welding High-Strength Steels in Pipe Applications

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 welders.

Choose the right filler metal

Welding high-strength steel requires using high-strength filler metals. When choosing a filler metal for these materials, it’s very important to match its strength to the rated strength of the base material. The maximum allowable strength for all grades of pipe is 110 ksi except for X80 pipe, which is 120 ksi.

Many new pipes are richer in alloy content and higher in strength due to newer fabrication procedures. Care should be taken when inspecting the material test reports to make sure that filler metals are not undermatching the mechanical requirements of the pipes or fittings being welded.

Overmatching the material strength can also be an issue, but it tends to be less common than undermatching. The higher the strength of a filler metal, the lower its ductility. Using a filler metal that is overmatched by too much with the base material can reduce weld ductility, increase hardness and make it harder to produce a weld with high toughness levels.

It’s also a best practice to choose a filler metal with low hydrogen content to reduce the chance of weld cracking. While stick welding remains common in many pipe welding applications, a move to a wire welding process like pulsed MIG or Regulated Metal Deposition (RMD®) using solid, metal-cored or flux-cored wires will help reduce hydrogen in the weld, as well as delivering improved productivity and lower heat input.

Filler metals for high-strength steels in pipe

The appropriate filler metal solution can vary depending on the pipe welding application and grade, so many variables need to be considered.

Some users may prefer to use a 90 or 110 ksi gas-shielded flux-cored wire while others may be using a 70 or 80 ksi metal-cored wire. Each pipe grade, location of the weldment and contractor process will dictate what products are used along with taking into account the mechanical requirement needs.

Whether you’re using stick welding or a wire process, the puddle appearance and behavior when welding high-strength steel isn’t much different from the puddle when welding mild steel.

Tips for welding high-strength steels:

• Process switch: More contractors have made the move in recent years to wire processes like RMD for root passes and pulsed MIG for fill and cap passes with proven results — especially in Canada. RMD is used in the downhill progression and is a synergic process that makes it easier for welders to make adjustments. It’s also more tolerant of contact-tip-to-work movements during welding.

• Root pass process: RMD for the root pass — even on high-strength steels — allows welders to put in high-quality root passes with less training time, less spatter, less heat input and thicker beads compared to using traditional stick electrodes. RMD is also a low-hydrogen process and gives welders good puddle control in all positions and better bead profiles in the overhead position. When using the RMD process, it’s recommended to use Bernard® Centerfire™ consumables.

• Choose the right gas: The shielding gas type will be determined by the weld procedure. Common gases for these steels range from a 90% argon/10% carbon dioxide mix to a 75% argon/25% carbon dioxide mix, with a flow rate of 35 to 40 cubic feet per hour (cfh).

• Pulsed MIG: Some operations are also having success using pulsed MIG for hot passes as well as fill and cap passes. It is a low-hydrogen process with lower heat input, little to no spatter and good puddle control when welding out of position.

Success with high-strength steels

High-strength steels offer benefits for toughness and durability that can be critical in many pipe welding applications. But to achieve the best results when welding these materials, take care to properly control the heat input and make sure to select the right filler metal to produce high-quality welds.