Weld Strength vs. Weld Toughness

If you have ever been tasked with selecting the correct specification and grade of steel for a particular application, you know there are hundreds of choices.?Typically, these are based on the material's tensile strength and yield strength.?Many factors go into selecting the suitable base metal for a given application.?In some cases, we want high strength, as with semi-trailer frames.?This is advantageous because it reduces the vehicle's weight by requiring a lesser cross-sectional area than using lower-strength steel to handle the same amount of stress.??In other cases, we want lower-strength steel that is more ductile and can absorb more energy before fracturing, as may be the case in some steel bridges.

Tensile strength, yield strength, and ductility are mechanical properties of steel.?Another mechanical property is fracture toughness, commonly referred to simply as 'toughness.'??In some cases, the terms toughness and strength are used interchangeably, but this is incorrect.?Instead, it is assumed that the stronger the material, the tougher it is.?Again, this stems from?a lack of understanding between strength and toughness.

To distinguish between the two, let's first look at some basic definitions.

Yield Strength?– a measure of the stress a metal can support before deforming.

Tensile strength?is the maximum stress that a metal can support before fracture begins.

Fracture Toughness?– the ability of a metal to absorb energy in the presence of a sharp notch, often when subjected to an impact load.

We probably want these three to be as high as possible for several applications.?There are specific design considerations why we may want the yield strength to be relatively lower than the tensile strength, but in general, the higher the strength, the better.?And based on the definitions above, we certainly want high fracture toughness if a crack develops or inspection missed other stress risers such as undercut, overlap, or incomplete penetration.

However, there is a problem in achieving high values for all.?There is an inverse relationship between strength and toughness for the base metals and filler metals used in steel construction.?As strength goes up, toughness goes down, and vice versa. The chart below illustrates this relationship.

Because strength (yield and tensile) is inversely related to toughness, we need to find a good balance when designing structures requiring a minimum fracture toughness.?A perfect example is that of a bridge.?Bridges endure a very harsh environment, from cyclic loads due to vehicles, wind, and water currents to freezing temperatures and corrosive environments.

It is almost a certainty that all steel bridges will develop cracks. Because of this, we want high fracture toughness to increase the time between crack initiation and brittle fracture, which could potentially be catastrophic. This is the same as decreasing the crack's growth rate.??This is very important in bridges because inspections are done at certain time intervals.?So, you don't want a crack to develop between assessments and propagate to the point of fracture.

A thorough understanding of the toughness of the base metal, weld metal, and heat-affected zone allows inspectors and engineers to determine if a crack on a bridge needs to be repaired or if it can be left alone for some time.?Repairs are incredibly costly.?If it is known that it will take a crack five years to reach a critical size, the determination to wait may be acceptable. But why wait to repair a crack??In some cases, it is best to do many repairs at once. So instead of repairing cracks on a bridge throughout the year, you do it once a repair is critical and take care of all cracks and other defects at that time.?This way, interruption to traffic is minimized, and costs are significantly reduced. All this while still guaranteeing the structural integrity of the bridge.

If we know that high toughness is critical, we will pick a material that has very low strength, right? Maybe not. If you go with lower strength steels, you lose the stiffness and strength required for some designs causing you to need a lot more steel.?This can be more support beams, thicker sections, etc.?On the other hand, using low-strength steel to benefit from higher toughness can mean increased costs in the manufacturing process.?As you can see, there is a delicate balance between the two.

The welding process can significantly change the toughness of base metal.?Because of this, codes such as the AWS D1.5 Bridge Code require CVN (Charpy v-notch) testing of welding procedures.?CVN testing provides values for toughness.?Manufacturers of filler metals are needed to do this testing when manufacturing consumables used in applications requiring minimum toughness values.?The contractor (fabricator) is then required to qualify welding procedures by testing, including CVN tests.