Rapid Stress Relief and Tempering: process description

Mario GRENIER and Roger GINGRAS E-mail adresses: mgrenier@pyromaitre.com and gingrasroger@videotron.ca Stress relieving of cold and tempering of hot wound springs is a century old process governed by fixed time temperature recipes. Ovens are standardized around temperature (heat) and not around time(transfer).Soak time therefore garantees uniform heat transfer. Better transferring ovens get penalized following fixed recipes. Using high speed heat transfer and a method to control it, much better uniformity and several folds cycle time reduction can be achieved. This paper will present the science behind the technology, Pyrograph heat transfer simulation, and where technology is used for in the most demanding stress relieving and tempering applications

Stress Relieving Process

Stress relieving is used to remove residual stresses which have accumulated from prior manufacturing processes. Stress relief is performed by heating to a temperature to achieve the desired reduction in residual stresses and then the steel is cooled at a rate to sufficiently slow to avoid formation of excessive thermal stresses.

Little or no stress relief occurs at temperatures < 260°C and approximately 90% of the stress is relieved at 540°C. The maximum temperature for stress relief is limited to 30°C below the tempering temperature used after quenching.1) Stress relieving results in a reduction

of yield strength in addition to reducing the residual stresses to some “safe” value and crack-sensitive materials. Typically, stress-relieving times for specific alloys are obtained from standards such as those listed in Table. 1. The stress-relieving times shown were developed for conventional convection heated batch ovens. With rapid stress-relieving technology the total stress relieving time for 16mm diameter CrSi wire can be reduced to 10 minutes or less.2-4)

Table 1 - Stress Relief Temperatures and Times for Wire

Material Specifications Temperature ( oF) Time (minutes) Music Wire ASTM A 228 450 30

Music Wire–tin-coated ASTM A 228 300 30 Music Wire Cadmium-Zinc

Coated ASTM A 228 400 30

Music Wire AMS 5112 540 60 O.T.M.B. ASTM A 229 450 30

H.D.M.B. Class I or II ASTM A 227 450 30 High Tensile Hard Draw ASTM A 679 450 30

Galvanized M.B. Class I or II ASTM A 674 450 30 Chrome-Silicon SAE J157 or

ASTM A 401 700 60

Chrome-Silicon (Lifens) SAE J157 725 60 Chrome-Vanadium ASTM A 231 700 60 Stainless Steel 301 ---- 650 30 Stainless Steel 302 AMS 5688 650 30 Stainless Steel 304 ASTM A 313 650 30 Stainless Steel 316 ASTM A 313 600 60

Phosphorous Bronze Grade A ASTM B 159 375 30 Hasteloy C ---- 500 30 Monel 400 ---- 625 60 Inconel 600 ---- 850 90

Inconel X 700 Spring Temper AMS 5699 1200 240

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Tempering process: When hot wound spring is hardened, the as-quenched

martensite is not only very hard but also brittle. Tempering, also known as “drawing”, is the thermal treatment of hardened and normalized steels to obtain the desired mechanical properties which include: improved toughness and ductility, lower hardness and improved dimensional stability. During tempering, microstructure modifications (carbide decomposition and martensite alterations) allow hardness to decrease to the desired level. The extent of the tempering effect is determined by the temperature and time of the process.5,6)

The tempering process may be conducted at any temperature up to the lower critical temperature (Ac1). Fig. 2 illustrates the effect of carbon content and tempering temperature on hardness of carbon steels.6) The specific tempering conditions that are selected are dependant on the desired strength and toughness.

Figure 2

Correlation of carbon content of martensite and hardness of different Fe-C alloys at different tempering

temperatures.

Typically, tempering times are a minimum of approximately one hour. Thelning has reported a “rule of thumb” of 1-2 hours/inch of section thickness after the load has reached a preset temperature.1) After heating, the steel is cooled to room temperature in still air. The recommended tempering conditions, in addition to recommended heat treating cycles, for a wide range of carbon and alloy steels is provided in SAE AMS 2759.

Tempering times and temperatures may also be calculated by various methods. One of the more common methods is to use the Larsen Miller equation. The Larsen-Miller equation, as seen in Fig. 4, although originally developed for prediction of creep data, has been used successfully for predicting the tempering effect of medium/high alloy steels.4) High Speed Convection

Stress relieving and tempering may be performed in convection furnaces, salt baths or even by immersion in molten metal. Among these, convection furnaces are the most common and it is important that they be equipped with fans and/or blowers to provide for uniform heat transfer when heating the load. Typically, convection tempering furnaces are designed for use within 150°C - 750°C.

One of the most important and critical deficiencies of most conventional stress-relieving and tempering ovens is the actual temperature non-uniformity of the material being heated. This is illustrated in Fig. 3 where it is shown that the actual temperature of the part depends on its placement in the basket and to a lesser extent on the belt in the oven.

Figure 3

Illustration of temperature non-uniformity present in conventional ovens.

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Ovens are standardized around temperature, not around time (transfer). Soak time at temperature is essentially the time necessary for each and every section of the load to reach process temperature. Most efficient ovens get therefore penalized. With controlled heat transfer, long soaking at temperature is not needed. Cycle time can also be reduced several fold. It is important to note that optimal stress relief for a given material is not a fixed value of one specific time and one specific temperature as suggested in Table 1. It is a temperature time function where temperature is the most important parameter. The faster the process, the more precision and control is needed

Figure 4

Larsen Miller function and different technologies respective domain.

Conventional (hours) - Pyro (minutes) - Induction (seconds)

Controlled Heat Transfer: Pyromaitre has developed over the years heat transfer

simulation software “Pyrograph” and “Pyrotemp” in order to optimize stress relieving and tempering process.

Both programs are based on “Heisler” unsteady state heat transfer equations.5) Parameters such as diameter, section thickness, alloy type and conductivity, part sizes, part weight, carbon content and hardness are put into the program to generate a surface and core temperature gain curve. Different curve patterns are then generated by the modifying time and temperature in the setup section. Software will automatically calculate stress relieving, or tempering temperature as a function of desired hardness or cycle time. Program will also calculate belt occupation ratio and power percentage for optimum usage of equipment. Program will also calculate operating costs.

Figure 5

Illustration of “PYROGRAPH” heat transfer and PYRO simulation software.

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Figure 6

Illustration of “PYROTEMP” heat transfer software to simulate tempering process in a PYRO. high speed high precision oven.

Pyro softwares are stress relieving, tempering and heat transfer science brought on the shop floor. Software are initially used to select proper equipment and then to set it up to it's maximum operating efficiency. Following are examples of practical results

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Figure 7 - Temperature profile of car suspension springs. 13,5mm CrSi. Stress relieved 440 oC in 6 minutes total

cycle time. Temperature spread left, center right, at exit as measured by contact thermocouple is smaller than <10 oC. Belt width 1,3 meter.

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Figure 8 - Heat transfer curves of rapid tempered clutch diaphragm springs. 6.5 minutes total cycle time. About all spring types being manufactured have been high speed stress relieved at this point in time all with 4 – 5 fold cycle

time reduction most of the time under 10 minutes and with significant product quality improvement in several cases.

High speed stress relieving and product quality: The number one reason for high speed stress

relieving is superior quality and product consistency. Over 15 years ago when the technique was developed, we were not after Olympic speed records. Objective was to meet a costumer most stringent 0 defect uniformity specification. By elevating the oven temperature, and shortening the cycle time, we discovered that consistency improved. Speed then

became a mean to reach customer requirements and became thereafter the most striking feature of the technology but always a mean, not a goal in itself.

Very often though, spring makers will say, since we are using higher temperatures, that for certain high strength CrSi alloys there is a risk of affecting tensile strength and will be nervous about the technology.

Tensile Strenght

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Figure 9

Effect of time and temperature on tensile strength. Institute of Spring Technology (IST)

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We have observed tensile strength is also a time temperature function. As you can see Fig. 9, thirty (30) minutes at 398oC is as damaging as three (3) minutes at 440oC

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Temperature and tensile strength is therefore not an absolute value but a time temperature function. The shorter the exposure the higher the temperature that can be tolerated. This relation fits well with the technology as demonstrated in Fig. 10 and Fig. 11.

Rapid Stress Relieve Study

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As Coiled

Figure 10

X-ray diffraction residual stress results on high speed stress relieving trials.

Strength Drop in the Stress Relieve Processes Rapid Heating Tests

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Figure 11 - Tensile strength as a function of time and temperature.

Nine (9) minutes at 460oC yields equivalent tensile strength drop as current process 45 min at 420oC. Seven (7) minutes at 440oC and five (5) minutes at 460oC yields equivalent residual stress results as current process

and better tensile strength values thus a gain in quality. The seven (7) minute cycle time process was selected here by this manufacturer of high precision springs.

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Tempering oven and cooler unit – ‘Parabolic leaf springs’ 30mm cross section (45 minute tempering cycle time)

P-1611G – ‘Engine valve springs’ 6000 pieces per hour (5 to10 minute cycle time)

P-1611EM – ‘Automotive CV joints’ 150 pieces per hour (6 to 8 minute cycle time)

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‘Automotive rear axle tempering unit’. (20 minute cycle time) Heating--cooling.

Currently, various mechanical parts have been high-speed tempered. These include automotive axles, hypoid gears

CV joints, bearing races, connecting rods, crank shafts, and connecting rods. CONCLUSIONS

In this paper, an overview of the metallurgy of stress-relieving and tempering has been provided. This discussion included a brief review of methodologies used at the present time to calculate stress-relief and tempering times. It was shown that although soaking times and temperatures were generally fixed by the steel chemistry, substantial reductions in process times can be achieved by accelerating the heat-up time by designing more efficient heat transfer between the heated atmosphere and the load by using high-speed convective, turbulent flow which also provides for significant improvements in temperature uniformity throughout the load. The use of a heat transfer simulation (Pyrograph -- Pyrotemp) to facilitate the process design was described. Selected examples illustrating successful high-speed stress relief were also provided. From this discussion, it is clear that substantial process design efficiency and property improvements which utilize less floor space and provide for greater production productivity are possible using a high-speed stress relief or tempering process.

REFERENCES 1). K-E. Thelning, in “Chapter 5:‘Heat Treatment –

General”, Steel and Its Heat Treatment – 2nd

Edition, 1984, Butterworths, London, pp207-318. 2) M. Grenier and R. Gingras, exposé titled “High

Speed Stress Relief”, at Proceed SMI Tech. Symposium, Chicago, IL, May 1999, pp125-128.

3) M. Grenier and R. Gingras, exposé titled “Advances in High Speed Stress Relief”, at Proceed SMI Tech. Symposium, Chicago, IL, June 2001, pp100-103.

4) M. Grenier, in “High Speed High Precision Stress Relieving”, Springs, October 2002, Volume. 41 No 5, pp68-71.

5) M.A. Grossmann and E.C. Bain, in “Chapter 5 – Tempering After Quench Hardening”, Principles of Heat Treatment, 1964, American Society for Metals, Metals Park, OH, pp129-175.

6) G. Krauss, in “Tempering of Steel”, Steels: Heat Treatment and Processing Principles, 1990, ASM International, Materials Park, OH, pp206-261.

7) D.J. Naylor and W.T. Cook, in “Heat Treated Engineering Steels”, Materials Science and Technology, 1992, Volume 7, pp435-488.

8) J.P, Holmon, in “Heat Transfer,” McGraw Hill 6th Edition 1986, New-York; pp136-152, 659-663.

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