Solid Height - Spring Manufacturers Institute

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Figure 2: Theoretical relationship between time after coiling and ID stress cracking for CrSi grade helical springs. also reduces the unwanted residual tensile stresses at the inner radius of the hook bends. When made and heated properly, extension springs can maintain initial tension after the stress relief, even those made with 17-7PH that is heat treated (aged) at 900° for one hour. Chrome Silicon (CrSi) Grades Due to the high residual ID stresses developed after coiling this crack-sensitive alloy, a delay in the application of the stress relief can cause the formation of cracks at the ID. Sources of this crack sensitization may be from internal or environmental exposure to nascent hydrogen. Typical steel produced from a conventional electric furnace operation contains residual hydrogen levels that can vary from about 4 ppm to 10 ppm. Degassed steel can attain hydrogen levels that are closer to about 2 ppm. High residual levels of nitrogen, tin, sulfur and phosphorus can also increase the sensitivity of hydrogen attack. Environmental exposure to moisture may also be a source of hydrogen embrittlement or stress corrosion [1]. One of the most frequently asked questions I receive is, “How long after coiling should I stress relieve chrome silicon springs?” Industry and automotive specifications alike use the phrase “immediately after coiling” to answer this question. That’s the safest answer to the question. But another answer is, “It depends.” Some CrSi springs can go for days before any cracking occurs, depending on many factors, some of which are listed in the above paragraph. For a particular non-degassed heat of steel, I’ve created the graph in Figure 2, above, to show a possible relationship between ID crack occurrence and time after coiling. I’ve designated two variables, t S and t L, for the time to initiate cracking for smallindex and large-index springs, respectively. With all processing and environmental variables being equal, it would be interesting to know what an acceptable delay time is for small-index CrSi springs. For steel with high levels of residual hydrogen, it is more likely 42 SPRINGS July 2006 that this delay time is measured in minutes than several hours. Age Hardening For most spring materials, there are two types of aging that can occur: strain aging on cold-worked steel and precipitation aging on some stainless steels. It is important to differentiate between these two mechanisms, as the application and effect on the material are very different. For cold-worked steel like music wire, a significant increase in the yield point can be gained by heating at a relatively low temperature. The severe deformation of the metal during drawing causes the elements, like nitrogen and carbon, to be uniformly distributed throughout the metal matrix. After the application of a stress relief in the range of about 400° to 700°F, some of these elements are able to migrate to specific areas in the microstructure, which causes the yield point to increase. This increase in yield strength is called “strain aging” and is accompanied by a decrease in ductility [2]. Precipitation-hardened stainless steels, like 17-7PH, are age hardened at a very specific temperature and time. The CH900 condition, for example, requires 900°F for one hour, as this heat-treat cycle produces the best combination of properties. Heat treating this grade at a temperature much greater or less than the prescribed 900°F isn’t recommended because the ideal properties may not be attained. Dimensional Stabilization Some spring dimens ions will change over time if not stress relieved. After stress relief, hard-drawn steel springs tend to increase in length and decrease in diameter, while stainless steel spring dimensions respond to heat by moving in the opposite direction. Springmakers must understand and be able to predict these movements to make high-precision springs. Figure 3, right, shows two torsion springs. The one on the left was not stress relieved, and the darker one to the right shows windup arm movement as a result of an in-line stress relief. Some springmakers use the arm’s position on a torsion spring as Figure 3: Two torsion springs. The darker one on the right has been stress relieved. a measure of the thermal effect of a stress relief. This appears to be common practice for carbon steel grades, and it can be used to relate a batch oven process to a high-temperature in-line furnace cycle.
Figure 4: Hypothetical relationship depicting the amount of stress relief vs. time for steel springs. Stress relaxation can be critical for some hightemperature applications. Stress relieving cycles can be optimized to minimize relaxation. An old rule of thumb is that the stress relaxation temperature should be at least 100°F higher than the maximum service temperature. Practical Application Springs are typically stress relieved using batch ovens or in-line conveyer furnaces. The cycle time for in-line or high-heat-transfer furnaces can be shorter than for the batch furnaces. This is accomplished by increasing the heat transfer in the furnace and by taking advantage of the time/temperature thermal relationship evident in particular grades of steel. Larson and Miller created a mathematical model for this effect in order to produce valid test results that substituted long test times for elevated test temperatures. Their model proposed that many different time/temperature combinations existed that could produce a similar thermal effect. For stress relieving steel springs this relationship may not be perfect, but it can be used as a good starting point to help predict the proper stress relief cycle. Most stresses are relieved in the first minutes after the steel is at temperature. For one particular temperature, a theoretical relationship is shown in Figure 4, above. One of the basic assumptions of the Larson- Miller parameter is that no structural changes occur in the metal over the range of temperatures being examined. Certain temperatures on some alloy systems may produce a significant metallurgical change that may affect the slope of this relationship. When this situation occurs, experimental data should be used [3]. For alloys like 17-7PH that can be age hardened at a very specific temperature, the Larson Miller equation would not apply for temperature ranges around 900°F. In general, it’s a good idea to stress relieve all springs in a timely manner, and many springmakers do this. However, to properly stress relieve springs fabricated from different materials or containing severe bends, it is important to realize that one size doesn’t fit all. Be on the lookout for that occasional spring that encounters “a few” failures every year. Does this spring have a low index? Is the material chrome silicon? For torsion springs, does it tend to crack at the bends in the arms before plating? If you can answer “yes” to any of these questions, consider reviewing the stress-relief process to ensure that it fits the application. The author thanks Loren Godfrey, president of Colonial Spring in Bristol, CT, for consulting on this article. For further information on the stress relief of springs, refer to Mark Hayes’ “Cautionary Tales” Parts VIII and IX, which appeared in the December 2001 and February 2002 issues, respectively. References 1. Parrington, Ron, Hydrogen Damage and Embrittlement, ASM Handbook, Volume 11 Failure Analysis and Prevention, Editors: W. Becker and R. Shipley, ASM International 2002, pp. 809-811. 2. Dieter, G.E., Mechanical Metallurgy, Sanjeev Rao, McGraw-Hill, 1986, pp. 201-203. 3. Dieter, G.E., Mechanical Metallurgy, Sanjeev Rao, McGraw-Hill, 1986, pp. 461-462. v Don’t Let OSHA Play Games with Your Company! Hidden Hazards can be Dangerous and Expensive! SMI’s On-Site Safety Audit will identify safety hazards and recommend abatements . The confi dential audit includes a preliminary interview, a review of the company’s written programs, a plant fl oor inspection and a detailed, written, post-audit report. Cost: $1,000 for SMI Members $1,250 for Nonmembers For more information, contact Jim Wood, SMI Regulations Complaince Consultant Phone: 630-495-8597 Fax: 630-495-8595 E-Mail: regs@smihq.org SPRINGS July 2006 43
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