Ulbrich Stainless Steels & Special Metals of North Haven, CT, uses a combination of exacting chemistry and precision processes to reduce variation in raw materials provided to the spring industry. These capabilities assist the springmaker in producing products with more predictable dimensions and performance, not the least of which is enhanced fatigue life. Though Ulbrich does not perform fatigue testing, we do control key variables, such as hardness, tensile strength or yield strength, that enhance the final product. An essential aspect of our role is to tailor the raw material to spring customer needs in a broad range of applications.

Type 301 Stainless Steel Options

Springs can be made from a wide range of temper-rolled or heat treatable alloys. Within a steel grade, we can control or enhance the most critical properties for the specific end use. Type 301 stainless steel, for example, is particularly useful for high-strength room-temperature springs because of its ductility at high strength levels in the temper-rolled condition, with or without stress relieving. Following are some properties that can be controlled within the Type 301 grade:

This multihead slitter cuts several narrow widths at a time and re-coils the strip on oscillate-wound spools.

Before You Order Material, Answer These 8 Questions: 1. What is the bend severity in fabricating the spring? 2. Does the finished spring require a stress relief anneal? 3. Is fatigue resistance critical to performance? 4. What is the operating environment of the spring: temperature, corrosive environment, abrasive environment? 5. Is the edge of the raw material also the edge of the spring? 6. Is the raw material to be supplied as oscillate-wound or ribbon-wound? 7. Which mechanical properties are most critical: yield strength, tensile strength, elongation or hardness? 8. Would a smooth or rough finish be advantageous? Providing this key information can help your supplier ensure that you receive the material best suited to your needs.

Left and above: Requirements for strip that must be truly flat are met using the latest in stretch-bend leveling technology. This leveler utilizes rugged tension bridles and the nest of small-diameter bending rolls, as shown in the above photo.

Above: Examples of constant-force springs manufactured with Ulbrich material.

Yield Strength: By adjusting the chemical composition and amount of cold rolling, a range of options can be provided in temper-rolled Type 301, from one-eighth hard to super hard. The yield strengths can range from 60 ksi up to 290 ksi, depending on the application. Once determined, these chemical compositions are provided consistently. The amount of cold reduction is slightly modified to accommodate minor changes in chemistry from heat lot to heat lot. Such modifications ensure that the springmaker gets material with the desired mechanical properties.

Ductility: At the highest strength levels, there are chemistry options to improve ductility. In fact, the limiting factor in increasing the yield strength is loss of adequate ductility. Using several options in chemistry and processing creates the optimum balance in properties to suit the needs of the individual customer.

Gauge Tolerance: Limiting gauge variation is accomplished by rolling the strip on Sendzimir mills or four-high mills with exacting automatic gauge controls. These controls, combined with precise laboratory gauging systems, enable material to be rolled to ordered nominal gauges with tolerances as low as ± 1% of nominal gauge for the most exacting applications. Needless to say, the selection of appropriate inventory, the use of meticulous techniques and precise inspection add to the cost of holding the tightest gauge tolerances. We recommend that these options be reviewed carefully to determine how tight the gauge tolerance should be specified. We can offer ± 1% for the most demanding applications, so that option is available for consideration as well as less stringent and less costly requirements.

Stress Relief: Ulbrich stress relieves temper rolled 301 stainless steel using either continuous furnaces or bell furnaces. This process increases yield strength, tensile strength and hardness, while reducing elongation. It is helpful if the purchase order or specification clarifies whether the metal is to be stress relieved or not. This issue must be handled effectively between material suppliers and springmakers to arrive at a process that often has substantial influence on fatigue properties.

Flatness: In strip, flatness is controlled using a combination of rolling and leveling technology. On light-gauge material, less than 0.025 in., with tensile strength below 250 ksi, we can use stretch-bend leveling. Critical flatness above 0.025 in. thickness is obtained by precise inventory selection and rolling practice. In either case, the goal is uniform flatness with a minimum of residual stress.

Camber: Camber is normally the result of out-of-flat material prior to slitting. In some cases, material can be reworked after slitting to reduce camber, but this option is not always possible. When camber is critical, the material must be flat before slitting.

Bend factor: Bend properties can be critical in forming springs. In these situations, the optimal chemical and mechanical properties can be engineered to enhance the total performance of the material. This is done through adjustments in the rolling process and chemical composition. Proper planning for this issue prevents unnecessary lost time and money on trials with inherently little chance of success or repeatability.

Springback: Controlling springback can be critical in stamped parts. This issue is normally addressed through tight chemistry specifications within the specific steel grade and a small range of acceptable yield strength.

Edges: Spring steels are often ordered with de-burred or skived edges. In addition, the coils may be traversely wound to increase coil size and productivity. The Ulbrich Specialty Strip Mill has just installed a state-of-the-art edging and oscillating line to address both of these issues.

Other Material Options

In addition to Grade 301, springs are made from other stainless and special metal alloys. These include stainless steel grades 302, 304, 17-7PH and A2861, and Inconel2 718 and X-750 nickel alloys. Except for the first two, these alloys can be heat treated to achieve high yield strengths. The range of temperatures in which they can be used varies; some are well suited to much higher temperatures than others.

Notes:
1. A286 is a trademark of Allegheny Ludlum Corp.
2. Inconel is a trademark of Special Metals Corp. group of companies.

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William Keenan is the director of metallurgy at Ulbrich Specialty Strip Mill in Wallingford, CT.

Keenan has worked in the steel industry for 26 years, 18 of which have been in the specialty steel sector. He is an active member of the American Society of Metallurgists (ASM) International.

“Ulbrich’s metallurgical and sales engineers develop specific order requirements with each customer,” says Keenan. “Our annual capital program will continue upgrading to higher productivity equipment, offering more options on finishes and recoiling methods, and investing in more sophisticated quality-control systems.”

Readers may contact him by phone at (203) 265-8204 or e-mail at wkeenan@ulbrich.com.