Designing with High Tensile Steel:

The majority of the design work Sparta does revolves around mobile equipment that is subject to strict weight limits due to road regulations. We are finding that our customers are demanding a more sophisticated product and asking us to make things stronger, lighter and of course cheaper. This is no easy task as there is a strong correlation between weight and cost in steel structures, as weight goes down cost generally goes up. However, one of the strategies we use to push our designs and meet these parameters is through the use of high tensile steel such as QT-100. Sparta has had a lot of success with this strategy and have employed it in many of our projects.

QT-100:

QT-100 is a grade of steel where the QT simply means quenched and tempered and the “100″ means 100,000 psi yield. QT-100 is the common name for this product but is also marketed under many other names such as T-1 or A514. The steel gains its characteristics from the process of quenching and tempering. This is a process steel suppliers will use to control grain structure of steel by heating the steel up to a specific temperature and controlling the rate of cooling in order to improve certain qualities(read more here: http://en.wikipedia.org/wiki/Heat_treating ). The resulting steel has improved strength and flexibility while still maintaining weld-ability. These characteristics have made it a fantastic tool for an engineer designing high performance equipment.

The Economics of High Tensile Steel

The economics of using high tensile steel in design are an interesting look at how engineers can add value to a project. QT-100 delivers a yield of 100,000psi which is approximately 2-3 times as strong as standard steel which ranges from 36,000psi for non structural to 50,000psi for structural grades. Using high tensile steel with this increased yield strength allows engineers to use steel that is ½ to 1/3 the thickness and weight to do same job. This by itself would be great, by switching to high tensile steel you have substantially reduced the weight of your product. In the transportation world less dead weight means more payload, more equipment and few truck loads for the customer all of which drive profit for the end user. The compounding effect of this is: at the time of writing mild steel with a 44,000psi yield costs about $52/CWT and QT100 is around $76/CWT.  So the high tensile steel is 50% more expensive but you can achieve the same strength using less than half the material. The end result is a product that is significantly lighter and even slightly cheaper.

Deflection and High Tensile Steels:

There are a few situations where having a high tensile steel doesn’t help at all. One of these are in cases where the design is dictated by deflection rather then yield. High tensile steel is able to deflect more then mild steel before breaking but both materials deflect the same amount with respect to a given load. In many building codes and crane applications deflection is the primary mode of failure. A common rule of thumb is  the allowable deflection in a steel structure is 1/300th or 1/600th the length in deflection. So a 600″ beam is allowed to deflect a maximum of 1″. When designing with these parameters the structure fails it’s deflection parameters long before it is close to yielding so a material with a higher yield but the same deflection is pointless. For instance you could build a walkway that was super light and made from high tensile steel but it would feel uncomfortable walking on it because it would be spongy. In another scenario, gantry style cranes are made from regular structural steel because you have to build them heavy enough that they don’t deflect to the point where it wont roll down the tracks anymore.

Other Forms of High Tensile Steel:

There are many other forms of high tensile steel often found in bar stock that has 100,000psi yield or even higher but this is achieved by adjusting the material composition. For instance 4140 is a high tensile steel bar stock with an increased carbon content but the increased yield comes at the expense of ductility and weld-ability.

High Tensile Design in Action

A good example of the application of these principles are with Sparta’s mast design. Traditional service rig masts are built by welding many 50,000 psi yield tubing in a criss-cross pattern up the length of the mast. In order to decrease weight and cost Sparta approached this problem a little differently then most companies. Using a mixture of high tensile plate and tubing to replace the traditional system Sparta was able to produce a fantastic product that is light, strong and easy to build.