Hot Setting
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| Pyro ContiQuench / InstaQuench hot-setting system |
How to improve long-term load stability
By Jim Demarest, Pyromaître and Derek Saynor, Spring Technology Consultants
Hot setting is a process often employed by springmakers to improve the long-term load stability of compression springs when it is anticipated that they will be exposed to elevated temperatures in service. Occasionally, if load stability is critically important, it is carried out on highly stressed springs that will operate at ambient temperatures but for extremely long periods of time. This article will discuss the hot setting process, applications, equipment and parameters, as well as how to calculate plastic set.
The Hot-Setting Process
Hot setting is usually carried out after the shot-peening
process and consists of compressing each spring to a fixed height while at an elevated temperature. This procedure causes the spring to suffer plastic set and shorten in length, thereby losing load during the process rather than in service. As a result, the hot-set spring subsequently loses load in service at a much reduced rate than a spring without the benefit of hot setting.
In order to maximize load stability in service, it is necessary to maximize the amount of plastic set in the spring during hot setting.
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| Pyro gas-fired hot-set oven. |
It is important to make a clear distinction between the plastic set and load loss that occur during the hot-set process, and the load loss that takes place in a hot-set test (sometimes called a “hot-oil test”). The hot-set test parameters and pass/fail requirements are often called out on the customer print or in a related technical specification. The purpose of the test is to verify that the finished spring has the required level of load stability necessary for correct operation in service. The test details and parameters are, however, very similar to the hot-set process itself, so it is important to be clear regarding the differences. The test usually requires the springs to be clamped to a fixed height and placed at an elevated temperature in a furnace for a period of several hours (usually one to 24 hours), and the load loss or loss in free length (i.e. the plastic set) compared against a maximum allowable value. In order to minimize the load loss during the test, it is necessary for the springmaker to maximize the load loss during the hot-set process.
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| Controlled and accumulating spring conveyance with external adjustment. |
Equipment
For hot-set applications that are low in volume and/or require minimal load loss in service, springmakers have traditionally employed a manual solution. The springs are clamped in a fixture to solid height, heated in a furnace for several hours and then cooled to room temperature prior to unclamping. This method requires a more modest capital investment, but is quite labor-intensive and energy-inefficient in that the mass of the springs and fixtures must be heated.
For hot-set applications that are high in volume, cost sensitive and require moderate load stability, springmakers employ an automated solution. The springs are heated in an oven, transferred to a press, clamped for a one to two seconds dwell, and quenched to room temperature prior to unclamping. This method involves a more significant capital investment than manual hot setting, and it requires skilled equipment operators. Most systems in operation today are custom solutions developed in-house with the support of outside machine shops or special machine builders.
To fill the market’s need for a standard fully integrated and automated system, Pyromaître developed two standard hot-set solutions from a common architecture: ContiQuench and InstaQuench. They represent the latest addition to Pyromaitre’s line of precision products, with the integration of Pyro high-speed ovens with a proprietary patent-pending hot-setting technique.
ContiQuench provides greater set times than conventional systems to deliver improved long-term load stability, without sacrificing throughput.
InstaQuench combines multiple quality-control operations into one package, minimizing in-process inventory and maximizing product traceability.
Applications
Typical hot-set spring applications include valve springs, fuel injector springs, clutch springs, transmission springs, pressure release valve springs and medical device springs. As operating-stress levels are increased and as equipment warranty periods are extended, the need for hot setting becomes greater.
Hot-Set Parameters
The main parameters that influence the amount of plastic set that can be generated during hot setting are as follows:
• Spring material type and grade.
• Residual stress in the spring.
• Spring design.
• Hot-set process parameters:
- Clamping stress.
- Clamping temperature.
- Clamping time.
Plastic set is often measured as the percentage reduction in free length during hot set. It is preferable, however, to refer to the torsional plastic strain (γ) that is locked into the spring as a result of the process. This parameter can be applied to calculate the plastic set across a wide range of spring designs, whereas the percentage change in free length is only valid for the test results carried out on a single spring design.
Oven speed and precision have proved to be key factors in minimizing load loss rejects. Several Pyro customers have reported reject rates of less than 0.5%, compared with 5-10% using conventional ovens. Furthermore, hot-set performance at the upper temperature limit further maximizes plastic strain reduction.
Spring Material Type and Grade
The intrinsic load stability of a compression spring is governed by the temper resistance of the spring material. In general, the ranking of material types (from best to worst) is as follows:
1. Nickel alloys.
2. Stainless alloys.
3. Oil-hardened and tempered silicon chrome.
4. Oil-hardened and tempered chrome vanadium.
5. Oil-hardened and tempered carbon steel.
6. Hard-drawn music wire.
7. Hard-drawn carbon steel.
8. Copper alloys.
For each material type, the actual load stability in service and the amount of plastic set during hot setting is also influenced by variation in chemical composition, heat-treatment condition and tensile strength of the material used. As a consequence, test results can show significant variation, depending on the exact condition of the wire used.
Residual Stress in the Spring
The state and magnitude of residual stress within the spring prior to hot setting has an important influence on the degree of load stability in service and on the amount of plastic set generated during the hot-set process. As a result, process parameters prior to the hot set can significantly influence the results obtained. The main factors that must be controlled to achieve consistent and predictable hot-set performance are:
• Effectiveness and efficiency of stress relieving after coiling.
• Degree of cold set during setting process.
• Intensity of shot-peening process.
Spring Design
Wire size, spring index and maximum solid stress are all important design factors that influence the load stability in service and the response of the spring to hot setting.
Pyromaître hot-setting systems feature contained and accumulating conveyance with easy external adjustment to minimize setup times.
Process Parameters
The following three parameters are the significant factors in the hot-set process that control the level of plastic set that occurs and, therefore, the subsequent load stability in service:
• Clamping stress.
• Clamping temperature.
• Clamping time.
• Clamping Stress
To maximize the amount of plastic set, the clamping stress should be as high as possible. However, in practice, this is limited by the solid stress of the spring being processed.
Frequently, for ease of setup and manufacture, the spring will be compressed to the solid height. However, for springs with an extremely high solid stress or with variable solid height, this can cause distortion in the part (bow and out-of-squareness) as well as inconsistent hot-set response (due to inconsistent solid stress). In these situations, the springmaker will usually clamp the spring at a height slightly longer than the maximum solid height anticipated in manufacture.
Clamping Temperature
It is also usual practice to maximize the clamping temperature in order to reduce the clamping time and maximize the rate of production. However, the maximum clamping temperature that can be used is limited for each material type to prevent tempering of the material, which would cause a catastrophic loss in material tensile strength.
For unpeened and uncoated springs, the maximum recommended temperature for hot setting should be no higher than the temperature used at the earlier manufacturing stage to stress relieve the parts after coiling.
For coated springs, the maximum clamping temperature may be further limited by the need to avoid thermal damage to the coating. The coating supplier’s data sheets should be consulted to establish this limit.
In the case of shot-peened springs, this maximum temperature is further limited by the need, at all costs, to avoid stress relieving the part. If this were allowed to occur, some of the beneficial residual stresses from the shot-peening process could be lost, with a consequent increase in the risk of fatigue failure.
The recommended maximum clamping temperature for shot-peened springs is listed for the most common material types below:
• Hard drawn carbon and music wire, 230°C (446°F).
• OT carbon steel, 230°C (446°F).
• OT chrome vanadium alloy, 250°C (482°F).
• OT silicon chrome alloy, 280°C (536°F).
• Stainless steel, 300°C (572°F).
• Data illustrating the influence of clamping temperature on the amount of residual torsional strain are shown in Figure 1, page 23. This data was generated from hot-set tests on shot-peened springs made from 3.8mm diameter, oil-tempered silicon chrome (UTS = 1700MPa) and chrome-vanadium alloys (UTS = 1560MPa), clamped at 1300MPa stress for one second.
Pyromaître hot-setting systems feature heated discharge chutes to maintain temperature during controlled escapement.
Clamping Time
Once the clamping stress and temperature have been fixed based on the spring design, the material and earlier processing, the required hot-set response can be selected by choosing the appropriate clamping time. Long clamping times will yield more plastic set in the spring and therefore give improved load stability in service (low relaxation). However, long clamping times traditionally also lead to low overall production speeds (increased machine cycle times). Consequently, springmakers have had to balance the conflicting requirements of improved performance and low manufacturing cost to arrive at an appropriate clamping time for the part in question.
Pyromaitre’s ContiQuench hot-setting system with the EverSet tooling package is designed to eliminate this balancing act, enabling high-speed processing with set times in excess of 10 seconds to volume-deliver an approximate 2 - 2.5 times greater reduction in plastic strain than conventional hot-setting methods. The combination of Pyro ovens and the ContiQuench setting system delivers hot-set performance at both the upper temperature limit and the maximum beneficial clamping time. ContiQuench allows springmakers to deliver improved long-term load stability at a reasonable cost.
Data showing the influence of clamping time on the amount of residual torsional strain are shown in Figure 2, left. This data was generated from hot-set tests on shot-peened springs made from 3.8mm diameter, oil-tempered silicon chrome (UTS = 1700MPa), clamped at 1300MPa stress.
Conclusion
Pyromaître’s ContiQuench hot-set system allows springmakers to maximize the level of plastic set achieved during hot setting to increase the long-term load stability of their products in service. In today’s competitive global marketplace, technology is the key ingredient in staying ahead of the curve. For the precision springmaker, that curve just happens to include the load loss curve.
Jim Demarest is sales and marketing manager for Pyromaître Inc., St-Nicolas, Quebec, with 10 years’ experience in the heat-treat industry. Demarest began his heat-treat equipment manufacturing career as a project manager and then sales engineer with Holcroft of Livonia, MI. He brought his expertise in custom integrated system to Pyromaître in 2005 and can be reached via the Pyromaître Web site, www.pyromaitre.com.
Derek Saynor is a Chartered Professional Mechanical Engineer with 30 years’ experience in spring technology. Having worked at the Institute of Spring Technology in England as a research engineer and then CEO, he moved nine years ago to take the role of senior vice president at Peterson Spring in Southfield, MI, with responsibility for engineering development activities. In the last year, he has set up his own consulting company to offer confidential technical services and training on the application and development of spring technology to solve their engineering problems. He can be contacted via his Web site at www.springtechnologyconsultants.com.
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