Design of Heat Treatment Cycles: A Case Study Stu dy for Salt Bath Ha Although heat treatment treatment operations operations have strong strong bearing on the final product product quality, trial and error methods. The present work describes an engineering approach to the d
Industry accepted heat treatment processing cycles might sometimes lead to longer p efficiency, efficiency, and higher energy consumption. This article elaborates elaborates on important engine austenitising temperature temperature and soaking time, selection of appropriate salt, rectification was successfully applied to an industrial scale heat treating operation for the producti Accurate ngineering !ompany "td. is leading Indian manufacturer of precision measu facilities in $une, India, the company offers a comprehensive range of measuring equi measuring fi&tures, and three coordinate measuring instruments. The heat treatment operation is an important step in the manufacturing of these preci product with stringent dimensional control as well as high wear-resistance. The comm equipment are: •
salt bath hardening operation, where the components are heated in a salt bath f
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cryogenic treatment for stabili'ation of martensitic microstructure, and
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tempering operation operation for obtaining a desirable combination of strength, hardness
Accurate ngineering recently procured a large amount of steel from (ermany at a ve used by one of the leading (erman slip gauge manufacturers. Although, Although, the compositi At the Accurate heat treatment shop, attempts were made to develop the heat treatm from + to /0! were observed during these trials, well below the required hardnes 2uring the past two decades, the process engineering group of Tata 0esearch 2evelop pro6ects on model based optimi'ation of various metallurgical operations. operations. The *rank ( this steel to obtain uniform hardness of /0! 1 after quenching and tempering operati
Approach to Design of the Heat Treatment Cycle /ardening of steel is achieved by transforming the ferrite7pearlite austenite phase to martensite phase by cooling. 8nder standard pr There will always be a minimal amount of retained austenite in the complete hardness, improve toughness and minimi'e distortion duri As depicted in *ig. 94a5, the transformation transformation to austenite requires h hypereutectoid steel 4with ! = .<5. 2uring the subsequent quen Fig. 1 chematic of transformation transformation to softer phases like pearlite and bainite. This is sch hardening operation. !"eft# Heating to the austeniti$ing temperature shown as shaded regime, and !%ight# high cooling rate for hardening.
The important metallurgical issues in designing a hardening cycle for tool steels are: •
>election of austenising temperature,
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Adequate soaking time for thermal homogeni'ation of the component,
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>election of appropriate quenching media to obtain required cooling rate,
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!ooling the component to the room temperature,
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Tempering temperature and time.
/owever, several other practical aspects, such as selection of salt and its neutrality ma These issues are elaborated in the following section.
Steel Grade Identification of the steel grade is the most important parameter fo *rank grade steel are tabulated in Table I. The composition falls in t is a shallow hardening tool steel and in rods above . inch in diam soft and tough core. These characteristics make it desirable for ma and is readily formable by forging. #? grade responds uniformly to
Selection of Salt for Process #hen selecting a salt for a given application, the following issues m •
The required heating temperature of the steel part must lie w
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The melting point should be low to avoid prolonged heat-up ti
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The salt must be compatible with quenching media% and
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The ease with which the salt is washed from the workpiece after heat treatment
At present, Accurate uses a proprietary @!-119 heat treatment salt, supplied by @at that its melting point is 9?? * 411 !5 and recommended working range is 9< to 9 as high as 9<+? * 49 !5. 8sing a salt above its recommended working temperature o&idation and decarburi'ation in the workpiece. Therefore, a suitable alternative had t Barium chloride-based salts are widely used for salt bath heat treatment of tool steels. salts are given in Table II. *or the *rank (rade steel, >alt C? with D Ba!l? and ?
Salt Bath Temperature & Soaking Time $roper control of salt bath temperature in the austeniti'ing range is lower temperatures will prevent the complete transformation of pea The time-temperature-transformation diagram for this grade is give 4D+? to D+ !5, so the recommended austeniti'ing temperature 4ba 9 * 4DD to < !5. *or comple& shapes and larger parts, it is r prior to hardening.
enable the recommended time of
As a best practice, uniformity surveys should be treatment temperature. These surveys are usual as illustrated in *ig. +. The soaking time in a salt complete phase transformation to austenite. "on soaking time in the salt bath furnace is ? to ? appro&imately 9 to + minutes for the parts in
Salt eutral salts used o&ides and the case of salt based salts 4>altC graphite rod for
ectification for austeniti'ing steel become contaminated with dissolved metals renders the bath o&idi'ing and bath furnaces with immersed electrodes, daily re 9,?,+ in Table II5 rectification should be done by one hour for every hours of operation.
Fig. & TTT diagram of A'' (& grade steel.
Fig. ) uggested locations for uniformity survey.
Tempering Tempering modifies the properties of quenched h and toughness. In general, two or more shorter retained austenite and for tempering the freshly suggested double tempering process for the *ran hardened tool steels like this steel should be tem temperature, to prevent or minimi'e cracking. !"P!#$!%TAT#% A%D !S'(TS Based on the items discussed above, the followin Fig. + chematic of hardness test Fig. * Typical cooling locations. curve for a Fig. Average and quenchant with three standard deviation of different stages of central points and cooling. corner for as-heat treated, .1 mm and .) mm ground samples.
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>alt Bath Temperature of 9<
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/olding Time of 9 minutes
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Fuenching in #ater
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Tempering at +E? * 4? !5 for minutes
*ifteen samples measuring mm & + mm & 9 mm were heat treated. The 0ockwell as at depths of .9 mm and .+ mm. The location where each hardness measurement measurements on the fifteen samples are summari'ed in *ig. 1. The mean hardness and standard deviation of the center point and the corners are plo the as-heat treated sample had an average hardness value of .< /0! with a standa variation is not acceptable in the final product. In the .9 mm depth sample, the avera deviation to E.<. The hardness value further improved to an average of 1.1 with a st hardness value is highly desirable for the slip gauges. It must be noted that the hardn company. The low hardness and wide variability of the as-heat treated samples was primarily du mm to .+ mm suggests decarburi'ation in the product. It is believed that the decarbu used at Accurate ngineering. It is interesting to note that in all the three cases, the center points showed a lower h be the result of a higher tendency of vapor blanket formation in the sample center as vapor blanket during the first state of quenching and could improve hardness uniformi !C$$!%DAT#%S The summary of recommendations as determined by the process analysis and the e&p •
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>alt mi&ture of D Ba!l? 7 + a!l should be used at an austeniti'ing temp 0ectification of the above salt must be done every four hours of operation with 9 in the bath for 9 hour.
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8niformity survey of the bath temperature should be conducted before loading t
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The workpiece should be cleaned from scale before heating in the salt bath furn
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The workpiece should be quickly transferred from the salt bath to the quench wa
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Fuenching water should be agitated in order to achieve hardness uniformity.
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#hen the part can be held by hand, it should be quickly transferred to the temp
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The tempering cycle should be repeated to achieve better dimensional stability.
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The parts should be cleaned in a solution of 9 aH/ by weight in water follo
S'$$A) By using the suggested heat treatment cycle determined for the *rank (rade steel, th was observed and attributed to the use of salt bath at higher than recommended tem
