March 10, 2011
Choosing the right spring for working with HSLA is very important. The author recommends chrome nitrogen springs of well-known brands for proper quality control.
Q: We are stamping a small bracket, about 1 in. wide by 2 in. long and 0.050 in. thick. Made of high-strength, low-alloy steel, it has a drawn feature on one end and multiple forms on the other. To meet the customer's requirements, we have to form down and draw up. Any advice?
A: This is a common question. First, when choosing springs, use this rule: If the requirements push the limits of the coil springs, always consider nitrogen springs first.
You will be out of luck if you design and build a tool that cannot accommodate the holding pressure you need to yield a capable part. The key word here is capable. Remember, it is easy to change the resulting dimension of a stamping by just changing the tooling; it is not possible to do this on a dimension that has unacceptable variation.
With that said, HSLA material is relatively difficult to work with on its own, and adding tooling requirements only makes it more challenging. HSLA steel has improved mechanical properties and greater resistance to corrosion than carbon steel. It is not made to meet a specific chemical composition, although there are guidelines and limits, but is produced to meet stringent mechanical properties.
The carbon content is between 0.05 and 0.25 percent to retain formability. Other alloying elements, including titanium, vanadium, and niobium, are added to maintain and increase the material's strength relative to the grain size. While they alter the microstructure of the steel and produce a very fine, even mix of carbides in the matrix, they do not help in the stamping process. With its high strength and toughness, HSLA steel is much harder to form than carbon steel.
For HSLA, choose compression springs that are cut to length, manufactured with flat ends, and ground so they seat squarely. Buying foot-long springs and cutting them down is inconsistent and not repeatable for maintenance over time.
When purchasing from a standard compression spring catalog, you can choose between carbon and chrome-vanadium styles. Carbon springs are cheaper and good for light duty, but they will not work as hard over time. I do not recommend these. Chrome springs are best for reducing downtime. Stainless and special-alloy springs also are available for environmental, magnetic, and special applications.
Of course, all compression springs are not alike. Some springs are manufactured and color-coded to look just like the top U.S. brands, but they are cheap knockoffs. Know where your springs came from. Top-quality springs are carefully processed through the following multiple steps; for the cheap imitations, some of these steps are not controlled, or they are left out entirely:
I have seen Belleville springs used in stacks for extreme applications, but I don't recommend it. They work best in static applications and will fail if repeatedly cycled.
Top-quality springs are essential to tooling performance. Know your supplier's process and buy direct. Saving a few dollars upfront will cost much more in production problems.
Good luck and happy stamping!