Design of Heat Treatment Cycles: A Case Study for Salt Bath Hardening of Tool Steel
Industry accepted heat treatment processing cycles might sometimes lead to longer product development time, sub-optimal level of operation at lower process efficiency, and higher energy consumption. This article elaborates on important engineering issues to be considered in salt bath hardening such as: selection of austenitizing temperature and soaking time, selection of appropriate salt, rectification of salt, tempering cycle and quality control measures. This holistic approach was successfully applied to an industrial scale heat treating operation for the production of slip gauges.
Accurate Engineering Company Ltd. is leading Indian manufacturer of precision measuring instruments and machine tools. With its state of art manufacturing facilities in Pune, India, the company offers a comprehensive range of measuring equipment including slip gauges, snap gauges, vernier calipers, air gauges, measuring fixtures, and three coordinate measuring instruments.
The heat treatment operation is an important step in the manufacturing of these precision instruments. Proper control during this operation is essential to obtain product with stringent dimensional control as well as high wear-resistance. The common heat treatments practiced during the manufacturing of these precision equipment are:
- salt bath hardening operation, where the components are heated in a salt bath furnace and quenched to room temperature,
- cryogenic treatment for stabilization of martensitic microstructure, and
- tempering operation for obtaining a desirable combination of strength, hardness and toughness.
Accurate Engineering recently procured a large amount of steel from Germany at a very attractive price. This grade of steel, locally branded as "Frank Grade," was used by one of the leading German slip gauge manufacturers. Although, the composition of this grade was provided, the heat treatment schedule was not revealed. At the Accurate heat treatment shop, attempts were made to develop the heat treatment cycle through trial and error method. Wide variations in hardness ranging from 30 to 45 HRC were observed during these trials, well below the required hardness of 60 HRC.
During the past two decades, the process engineering group of Tata Research Development & Design Centre (TRDDC) has successfully executed several industrial projects on model based optimization of various metallurgical operations. The Frank Grade problem was undertaken by TRDDC to design a heat treatment cycle for this steel to obtain uniform hardness of HRC 60 after quenching and tempering operations.
Hardening of steel is achieved by transforming the ferrite+pearlite phases to austenite phase by heating and subsequently by transforming the austenite phase to martensite phase by cooling. Under standard processes, it is difficult to obtain a 100% transformed martensite structure. There will always be a minimal amount of retained austenite in the structure. It is desirable to transform this retained austenite to ensure complete hardness, improve toughness and minimize distortion during the service.
As depicted in Fig. 1(a), the transformation to austenite requires heating above Ac3 for hypoeutectoid steel (with %C < 0.8) or above Ac1 for hypereutectoid steel (with %C > 0.8). During the subsequent quenching operation, the cooling rate should be high enough to avoid transformation to softer phases like pearlite and bainite. This is schematically shown in Fig. 1(b).
The important metallurgical issues in designing a hardening cycle for tool steels are:
- Selection of austenizing temperature,
- Adequate soaking time for thermal homogenization of the component,
- Selection of appropriate quenching media to obtain required cooling rate,
- Cooling the component to the room temperature,
- Tempering temperature and time.
However, several other practical aspects, such as selection of salt and its neutrality maintenance, need to be addressed for successful industrial scale hardening. These issues are elaborated in the following section.
Identification of the steel grade is the most important parameter for designing a hardening cycle. The nominal and specified composition of the Frank grade steel are tabulated in Table I. The composition falls in the AISI Type W2 grade (water-hardening high carbon tool steel). This grade is a shallow hardening tool steel and in rods above 0.5 inch in diameter hardens selectively providing a hard, abrasion resistance surface and a soft and tough core. These characteristics make it desirable for many tools that are subjected to impact during use. It has a good machinability and is readily formable by forging. W2 grade responds uniformly to normal heat treatments and is a widely used steel for many purposes.
When selecting a salt for a given application, the following issues must be considered:
- The required heating temperature of the steel part must lie within the working range of the salt;
- The melting point should be low to avoid prolonged heat-up times for heavy loads;
- The salt must be compatible with quenching media; and
- The ease with which the salt is washed from the workpiece after heat treatment and the affinity of salt for moisture must be considered.
At present, Accurate uses a proprietary MNC-661 heat treatment salt, supplied by Matador Chemical Industries, India. The specification sheet of this salt indicates that its melting point is 1220 F (660 C) and recommended working range is 1508 to 1580 F (820 to 860 C). However, the heat treatment requirement at Accurate is as high as 1832 F (1000 C). Using a salt above its recommended working temperature can result in oxidation of the salt as well as increase the possibility of oxidation and decarburization in the workpiece. Therefore, a suitable alternative had to be identified.
Barium chloride-based salts are widely used for salt bath heat treatment of tool steels. The typical compositions and recommended working temperatures for these salts are given in Table II. For the Frank Grade steel, Salt #2 with 70% BaCl2 and 20% NaCl is recommended for austenitization.
Proper control of salt bath temperature in the austenitizing range is important, as very high bath temperatures will result into grain growth while lower temperatures will prevent the complete transformation of pearlite to austenite.
The time-temperature-transformation diagram for this grade is given in Fig. 2. The Ac1 temperature for this grade of steel is 1345 to 1369 F (732 to 743 C), so the recommended austenitizing temperature (bath temperature) for this grade of steel should be in the range of 1428 to 1555 F (775 to 845 C). For complex shapes and larger parts, it is recommended to preheat the workpiece at 1202 F (650 C) for stress relieving prior to hardening.
Neutral salts used for austenitizing steel become contaminated with soluble oxides and dissolved metals during use. As the buildup of these oxides and dissolved metals renders the bath oxidizing and decarburizing toward steel, it is necessary to periodically rectify the bath. In the case of salt bath furnaces with immersed electrodes, daily rectification of the bath is required. For the recommended barium chloride-based salts (Salt# 1,2,3 in Table II) rectification should be done by adding 125 gm of boric acid (for each 100 kg of salt) and inserting a 3-inch graphite rod for one hour for every 4 hours of operation.
Tempering modifies the properties of quenched hardened tool steel and renders a desirable combination of strength, hardness and toughness. In general, two or more shorter tempering cycles are recommended for complete transformation of the retained austenite and for tempering the freshly formed martensite during cool-down after the first tempering cycle. The suggested double tempering process for the Frank grade steel calls for a 45 minutes treatment at 392 F (200 C). Water-hardened tool steels like this steel should be tempered immediately after hardening, preferably before they reach room temperature, to prevent or minimize cracking.
EXPERIMENTATION AND RESULTSBased on the items discussed above, the following heat treatment schedule was selected:
- Salt Bath Temperature of 1508 F (820 C)
- Holding Time of 15 minutes
- Quenching in Water
- Tempering at 392 F (200 C) for 45 minutes
Fifteen samples measuring 55 mm x 35 mm x 10 mm were heat treated. The Rockwell HRC hardness values were measured on the as-heat treated surface, as well as at depths of 0.1 mm and 0.3 mm. The location where each hardness measurement was taken is shown schematically in Fig. 5. The results of the hardness measurements on the fifteen samples are summarized in Fig. 6.
The mean hardness and standard deviation of the center point and the corners are plotted for each of the three locations. As is evident from Fig. 6, the corners of the as-heat treated sample had an average hardness value of 44.8 HRC with a standard deviation of 12.1. Such a low average hardness value and high degree of variation is not acceptable in the final product. In the 0.1 mm depth sample, the average hardness value increased to 51.4 along with a reduction of the standard deviation to 9.8. The hardness value further improved to an average of 60.6 with a standard deviation of 1.2 for the 0.3 mm depth. Such a high and uniform hardness value is highly desirable for the slip gauges. It must be noted that the hardness value of 30 to 45 HRC was obtained during the in-house trials at the company.
The low hardness and wide variability of the as-heat treated samples was primarily due to the surface scale, whereas the improvement in hardness levels from 0.1 mm to 0.3 mm suggests decarburization in the product. It is believed that the decarburized layer is due to the non-rectification of the MNC 661 salt currently being used at Accurate Engineering.
It is interesting to note that in all the three cases, the center points showed a lower hardness value as compared to the corner points. Such an effect is believed to be the result of a higher tendency of vapor blanket formation in the sample center as compared to the sample corner. Agitation in the quench bath would break the vapor blanket during the first state of quenching and could improve hardness uniformity.
RECOMMENDATIONSThe summary of recommendations as determined by the process analysis and the experimental results are given below:
- Salt mixture of 70% BaCl2 + 30% NaCl should be used at an austenitizing temperature of 1508 F (820 C) and soak time of 15 minutes.
- Rectification of the above salt must be done every four hours of operation with 125 gm of boric acid for every 100 Kg of salt and by inserting 3" graphite rod in the bath for 1 hour.
- Uniformity survey of the bath temperature should be conducted before loading the workpiece at desired temperature.
- The workpiece should be cleaned from scale before heating in the salt bath furnace.
- The workpiece should be quickly transferred from the salt bath to the quench water maintained at a temperature less than 95 F (35 C).
- Quenching water should be agitated in order to achieve hardness uniformity.
- When the part can be held by hand, it should be quickly transferred to the tempering furnace and held at 392 F (200 C) for 45 minutes.
- The tempering cycle should be repeated to achieve better dimensional stability.
- The parts should be cleaned in a solution of 10% NaOH by weight in water followed by rinsing in plain water.