Categories: Feature Stories Date: Jul 23, 2008 Title: Considerations for Welding Tomorrow’s High Strength Steels
In today’s manufacturing and fabricating world, speed is key. Companies strive to make more with less—less availability of skilled labor and less overhead—while still keeping pace with the demands of a technology thirsty society and increasing the bottom line.
Under these circumstances, tomorrow’s welding operators face a unique set of challenges, not the least of which is learning how to weld more, faster. They must also learn how to weld different materials, including high strength steels that are being used to meet greater production demands.
So, what will it take to meet these challenges? In a few words—knowledge, caution and time. Welding high strength steel requires more careful heat control and tougher filler metals than the mild steels that are more commonly used.
Understanding the Base…
As with any material, tomorrow’s welders should understand the chemical and mechanical composition of high strength steel as a first step toward welding success.
Due to their elevated strength-to-weight ratio, high strength steels are used to build products that carry more, weigh less and withstand extreme service conditions. Materials that meet these demands include HSLA (high strength low alloy) steels and AHHS (advanced high strength steels), with even more and stronger types of steels on the horizon.
HSLA steels in particular have been around for many years, but they have increasingly found a place in new industries because of their long-term cost effectiveness in high production applications. They are often used in heavy equipment manufacturing and structural steel applications as a means to lighten loads—these steels allow manufacturers to use thinner cross sections of material (as compared to A36, for example) for such applications, without sacrificing toughness or strength. Other HSLA applications include beams or frames for trailers, along with portions of bridges, crane booms and masts. On average, these materials provide yield strengths up to 85 ksi (586 MPa).
AHHS (advanced high strength steels) have also begun to emerge in recent years and offer even stronger mechanical properties than HSLA—as great as 140-ksi (965 MPa) yield strength. There are also reports of AHSS being developed that offer up to 160 ksi or more (1103 MPa +) yield strengths. AHHS are particularly useful in the manufacturing of automotive components, lightening overall vehicle weight while still providing the strength required for safety.
High strength steels gain their mechanical properties from, among other things, quenching and tempering. First, the material is quenched, a process by which the steel is rapidly cooled to room temperature. This is often done with a water spray or a special oil or gas method. During the tempering process, the steel is heated to a temperature that is just below its melting point for a specified duration of time. Typically, tempering ranges from a minimum of 300 degrees Fahrenheit (150 degrees Celsius) up to 1600 degrees Fahrenheit (871 degrees Celsius). Tempering duration will vary by material thickness and the desired properties.
High strength steels maintain a similar chemical composition to standard mild (or low carbon) steel throughout the quenching and tempering, but together the processes drastically increase these steels’ strength. This added strength requires tomorrow’s welders take special precautions during the welding process.
Beat the Heat…
High strength steels tend to be more sensitive to cracking than mild steels, which is why implementing good heat control prior to and during the welding process is crucial. First, preheating is a best step toward controlling the temperature gradient, or the range of temperature increase and decrease that occurs during welding. This is especially important, as welding on a cold piece of high strength steel will cause the material to heat up too quickly, and in turn cool too rapidly. This rapid cooling is the leading cause of cracking and preheating helps prevent it.
Pre-heating also helps reduce hydrogen levels and in turn minimize hydrogen-induced cracking. The proper pre-heat temperature is determined by the exact grade of high strength steel and its thickness. Welders should follow the recommended welding procedures for a specified application to determine the correct temperatures.
Likewise, welders need to maintain the correct interpass temperature for a given thickness of high strength steel. Doing so prevents a larger heat affected zone (HAZ) from forming during welding; HAZ is the area between the weld deposit and the base metal and an area that is prone to cracking. Maintaining interpass temperatures also reduces changes to the grain structure of the steel during cooling, which in turn limits any mechanical changes that could adversely affect the steels’ toughness or tensile strength. Welders can use contact pyrometers, Tempilstiks or other heat-sensing devices (including infared) to track the interpass temperature specified for their particular welding procedure.
As with any welding process, controlling travel speed and maintaining the recommended welding parameters (volts, amps) minimizes heat input when welding high strength steels.
Take Two on Filler Metals…
Choosing filler metals with the least amount of hydrogen content, as well as those with good toughness (high impact values) and the appropriate strength is key when welding high strength steels. Each of these features also helps prevent cracking.
For both cost and design purposes, certain applications require high strength steels to be joined to lower yield strength steels. Depending on the joint design and the load path (the area that will carry the bulk of the stress of a finished product) welders will often need to match the filler metal strength to the lower strength steel. As an example, when welding a HSLA steel (85-ksi yield strength) to A36 steel (36-ksi yield strength minimum), welders would choose a 70 grade flux-cored or metal-cored wire with 70 ksi tensile strength.
Note: base material is measured in yield strength, but filler metals are measured in UTS or ultimate tensile strength.
If a welder is welding high strength steels to each other, then typically the filler metal will match the yield strength of the two. For example, welding high strength steel with 100 ksi yield strength to itself would require a filler metal that offers a minimum of 90-ksi tensile strength (and a maximum of 120 to 130 ksi). Options for filler metals with these tensile strengths include low alloy metal-cored or low alloy flux-cored wires. Some solid wire and stick electrodes are also available.
Metal-cored wires offer the advantage of faster travel speeds (over solid wires or stick electrodes), which helps minimize heat input and with it, the potential for cracking and distortion. Most low alloy metal-cored wires also offer excellent toughness properties, and bridge gaps more efficiently than other filler metals. They also have low hydrogen levels.
Gas-shielded low alloy flux-cored wires are options for welding high strength steel to itself or to a lower strength material. Ones with a T-5 basic slag system are recommended due to their good mechanical properties and strength, plus they resist hydrogen pickup and can weld through light rust and mill scale. Wires with a T-5 slag system, however, are not as welder-friendly as those with a T-1 rutile slag system. Conversely, wires with a T-1 slag system have good arc stability and weldability, but they tend not to have as good of ductility and toughness. Like any other flux-cored wires, both produce slag that will need to be removed after welding or in between passes.
To date, there are only solid wires available for welding steel with greater than 120-ksi-yield strength.
Of course, there is more to welding high strength steels than just watching heat input and choosing filler metals. Still, these considerations are certainly a start. As the steel industry continues to push the boundaries of strength and toughness with materials, tomorrow’s welders will have to continue learning, training and keeping tighter welding parameters than ever before.