Distortion and residual stress development during quenching

In this column, I will discuss the different types of precipitation hardening stainless steels and discuss their physical metallurgy.

Introduction

The problem of distortion is universal across industry. A study by Thoben [1] indicated that the 1995 losses from heat treatment alone in the German machine, automotive, and transmission industry exceeded 850M € (1 billion USD). This does not include the rest of Europe, Asia, or the Americas. The problem is truly immense.

During quenching, there is a volumetric contraction as the work piece cools. As austenite transforms, there is a volume expansion. Martensite expands the most, as a function of the carbon content. Other phases can form, such as bainite or pearlite, which have much smaller volumetric expansion. Generally, outer fibers transform first, then inner fibers. This is further complicated by the effect of martensitic transformation start temperatures. Depending on the relative M_s temperatures and the thickness of a part, it is often possible that the core will transform first due to a much higher M_s temperature. As the remaining part transforms, a stress reversal occurs.

Quenching

A comparison of the distortion resulting from different quenching methods (traditional oil and high-pressure gas quenching) to the same hardness, and the racking method was conducted by Jurci [2]. Helical gears and mating pinions made from low carbon structural steels were gas- or low-pressure carburized to an effective case of 0.7 mm. The dimensional distortion (shrinkage and growth) and the shape distortion (roundness, flatness, and tooth deviations) were measured. They also measured the distortion as a function of position in the rack.

The authors found a significant reduction in distortion using gas-pressure quenching than the traditional oil quenching. However, they found that the use of nitrogen gas as a quenchant leads to higher out-of-flatness than oil quenching. They also observed a greater scatter in the results with gas quenching. This was attributed to inhomogeneity of gas flow within the quenching chamber.

Braun [3] reviewed the effects of racking and condition of the oil on the resultant distortion of steel components. He found that as an oil aged, greater distortion resulted.

In another study [4], the viscosity and cooling curve was evaluated. A single viscosity base oil, and different quantities of a high viscosity (450 cSt at 40°C) were mixed. A standard speed improver quantity was added to the mixtures. Cooling curves were measured using the JIS K2242 silver probe. They showed that the speed increased from 90°C/s to 100°C as the viscosity increased to approximately 100 cSt. However, they found that the surface hardness of S45C decreased from HRC 55 to HRC 29 as the kinematic viscosity increased. They attributed the improved quenching performance at low viscosities to the composition of the base oils having a low boiling point fraction.

A comparative study comparing the residual stresses and distortion in cylindrical components of AISI 5160H spring steel quenched at various temperatures of 25 percent poly-alkylene glycol (PAG) and compared to an accelerated quench oil was conducted by Sarmiento [5].

The results showed that no residual stresses were measured in cylinders quenched in either PAG or quench oil for diameters of 13.5 and 20 mm.

In a sensitivity study of quenching variables (orientation, slenderness ratio, M_s temperature, quench path and immersion rate), MacKenzie [6] examined the effect of the primary quenching variables on the hoop and axial stresses in simple cylinders. This study found that the orientation (horizontal or vertical) and the M_s temperature were the primary main effects for increasing radial and hoop stresses.

Racking in a vertical fashion increased the hoop stress, as did decreasing the martensite start temperature. Radial stresses were decreased by racking vertically, while decreasing the martensite start temperature increased radial stresses. The immersion rate was shown to have little effect on the residual stresses as the part cooled. The quench path, specifically the stability of the vapor phase, caused increased residual stress. A more stable vapor phase resulted in reduced residual radial and hoop stresses. This was thought to be due to decreased thermal gradients present. This result was unexpected and was thought to be associated with the alloy selected (41XX). This alloy has a large hardenability, so decreased initial thermal gradients would show less resultant residual stresses.

Arimoto [7] reviewed the status of quenching Fe-Ni alloys and modeling residual stresses. His work showed that transformation plastic strain is generally like plastic strain during heat treatment. Plastic strain is mainly produced during rapid cooling during quenching, while transformation plastic strain is due to martensitic transformation. Both the strain distributions show a tendency of the mirror symmetry especially in circumferential and axial components.

Liscic [8] indicated that there are three simultaneous processes occurring during the heat-treatment process. He also proposed a new quenching technology using both a volume of gas, and dispersed liquid nitrogen, to specifically control heat extraction and heat transfer to minimize distortion and residual stress. This method would be amenable to automated control.

Narazaki [9] predicted the heat transfer coefficients and applied them to a steel helical gear. Using the Japanese silver probe, an estimate of the heat transfer coefficient was determined by the lumped method. They found that the difference in material between the silver probe and the steel (SCr420H) gear affects the minimum temperature of the vapor blanket phase. The high conductivity of the silver rod probe resulted in stabilizing the vapor phase, with the result of lowering the minimum temperature of vapor blanket.

Funatani [10] reviewed the effects of quenchant and quenchant temperature on the distortion of SCM420 bearings 80 mm in diameter by 44 mm tall. As the heat extraction speed increased, hardness increased, but elliptical and cylindrical distortion also increased. As the temperature of the quenchant increased, hardness and distortion also decreased.

Racking and fixturing

The impact of racking and fixturing on 120 mm diameter 20MnCr5 (SAE 5120) gear blanks was investigated by Clausen [11]. In this study, gear blanks were carburized and quenched. During carburizing, the disks were supported by two-point loading or three-point loading. Parts were preheated at 850°C and carburized at 940°C to obtain a carburized depth of 0.8 mm. Quenching was accomplished by high-pressure nitrogen. Measurements were carried out before and after heat treatment. It was determined that the method of racking was the most critical factor affecting gear blank distortion (Figure 1 and Figure 2).

Base grids used to support work are expensive and must be replaced at routine intervals. To save money, maintenance of these grids is delayed or neglected. As these grids are used, distortion of the grid occurs. Takahashi [12] investigated the distortion of SCM420H helical gears with controlled hardenability. He found that flatness of the gears decreased as a function of time of use of the base grid (Figure 3).

Conclusions

The control of distortion and residual stress is a complex undertaking. Great strides have been made in the modeling and simulation of machining, heat treatment, and carburizing. However, the best modeling efforts can be undermined by variations in process. Dull tools, feeds and speeds, and even clamping methods result in changes and variations in residual stress and distortion. 

References

1. K.-D. Thoben, T. Lübben, B. Clausen, C. Prinz, A. Schulz, R. Rentsch, R. Kusmierz, L. Nowag and H. Surm, “Distortion Engineering,” HTM, vol. 57, no. 4, pp. 276-282, 2002.
2. P. Jurci, P. Stolar, P. Stastny, J. Podkovicak and H. Altena, “Investigation of Distortion Behaviour of Machine Components due to Carburizing and Quenching,” in IFHTSE 5th International Quenching and Control of Distortion Conference, Berlin, Germany, 2007.
3. R. Braun, “Influence of the Quenching Process on the Microstructure and Distortion of Heat Treated Parts with Particular Emphasis on Quench Oils,” in IFHTSE 5th International Conference on Quenching and Control of Distortion, Bremen, Germany, 2007.
4. Y. Tomita, K. Kukuhara, S. Asada and K. Funatani, “The Influence of Cooling Characteristics and Viscosity on the Hardening Behaviour of Steels,” in IFHTSE 5th International Conference on Quenching and Control of Distortion, Berlin, Germany, 2007.
5. S. Sarmiento, M. Castro, G. E. Totten, L. Harvis, G. Webster and M. Cabre, “Modeling Residual Stresses in Spring Steel Quenching,” in Heat Treating 2001 – Including Quenching and Distortion, Indianapolis, IN, 2001.
6. D. S. MacKenzie and D. Lambert, “Effect of Quenching Variables on Distortion and Residual Stresses,” in Proceedings of the 22nd Heat Treating Society Conference and the 2nd International Surface Engineering Congress, Indianapolis, IN, 2003.
7. K. Arimoto and K. Funatani, “Historical Review of Residual Stress in Quenched Fe-Ni Alloy Cylinders and Explaination of Its Origin Using Computer Simulation,” in Proceedings of the 24th ASM Heat Treating Society Conference, Detroit, MI, 2007.
8. B. Liscic, “History and Perspective of Controllable Heat Extration During Quenching,” in ASM 2001 Heat Treating Society Conference, Indianapolis, IN, 2001.
9. M. Narazaki, H. Shichino, T. Sugimoto and Y. Watanabe, “Validation of Estimated Heat Transfer Coefficients during Quenching of Steel Gear,” in IFHTSE 5th International Quenching and Control of Distortion Conference, Berlin, Germany, 2007.
10. K. Funatani, “Distortion control via optimization of cooling process and improvement of quench oils,” in IDE 2008, Bremen, Germany, 2008.
11. B. Clausen, F. Frerichs, D. Klein, T. Kohlhoff, T. Lubben, C. Prinz, R. Rentsch, J. Solter, D. Stobener and H. Surm, “Identification of Process Parameters affecting Distortion of Disks for Gear Manufacture – Part II: Heating, Carburizing and Quenching,” in IDE 2008, Bremen, Germany, 2008.
12. A. Takahashi, T. Morishima and H. Yamada, Journal of the Japanese Heat Treating Society, vol. 30, no. 6, p. 301, 1990.