1 Introduction Properties such as: hardness, impact strength, ten-sile strength and fatigue bending strength of carbonitridedparts of machines depend on austenitizing and quenchingprocess conditions. Quenching process conditions affectend-use properties such as: type and value of internalstresses, mechanical properties and the extent of harden-ing deformations<1>, <2>. The link between coolingprocess and properties of hardened carbonitrided compo-nents is microstructure, especially retained austenite andcarbides content.The scope of the use of aqueous polymer quenchantsto quench a wide variety steels has been published in nu-merous references<1>? <14>. Aqueous polymer solutionswill often result in improved as-quenched and temperedproperties at a lower cost than more traditionally usedmedia such as oil. Quench severity may be varied over awide range fromthatof a slowoil to agitated water ( H =approx. 0
·2 >1·0 ) by controlling concentration, agita-tion and bath temperature.Some of the problems encountered with quench oilsinclude: varying cooling rates, flammability, smoke andfumes. Water may also yield varying cooling rates, crack-ing and poor dimensional stability<15>. Cracking may beavoided by using oil because of its higher boiling pointand fluid viscosity which results in lower cooling rates inthe martensite transformation range for many steels. Gen-erally, heat transfer rates decrease with increasing viscos-ity. However, the favorable properties exhibited byquench oils may also be achieved with a polymer quen-chant by selecting the appropriate polymer and quenchingconditions (concentration, bath temperature and agita-tion)<16>.The quench severity of water can be decreased byraising the bath temperature which yields a correspondingincrease in film boiling range which often results in in-creased pearlitic structure. In addition to relatively highheat transfer rates, water is: not flammable, relativelylowcost, and non-toxic. However, the low boiling pointtional deficiencies of water are non-uniform wetting andcorresponding high thermal gradients and susceptibility tocontamination.Oil is a commonly used quenchant because it pro-vides optimum hardness with maximum dimensional con-trol in spite of its deficiencies. There are many differentquench oils available. A quench oil is typically classifiedas either a conventional ( slow) oil or an accelerated(fast) oil. In addition, there are high-temperature (hotor martempering) oils.Aqueous polymer quenchants offer an attractive al-ternative to petroleum oil. One of the greatest advantagesof aqueous polymer quenchants, especially aqueous poly-alkylene glycol (PAG) quenchant solutions, is that theyprovide excellent quench uniformity, especially relative towater. IA5% aqueous solution of a PAG quenchant hasbeen shown to provide uniform surface cooling relative towater with essentially the same centerline cooling rate.This means that a low concentration of an aqueous PAGquenchant can be used to replace water to eliminate thesubstantial thermal gradients exhibited by water which of-ten results in increased distortion and cracking.This paper describes the use of an aqueous polymerquenchant to harden carbonitrided steel. The results arecontrasted to those obtained with water and oil.2 ExperimentalA16HG (16MnCr5 according to ISO 683-11 1987;20CrMn according to GB 3077-88) chromium-manganesesteel (0.15%C, 1.55%Cr, 0·51%Mn, 0·20%Si,1·61%Ni) was used for this work. The carbonitridingprocess was conducted at850℃ for 5h with a carbon po-tential of 0·9% and nitrogen potential of0·4%Nusing avertical pipe furnace with forced atmosphere circulation.A detailed description of the experimental methodol-ogy for determining the microstructure, hardness, residu-al stress, and wear properties has been described previ-ously<12>and will not be discussed further here.The Tensi agitation device (flow cell) used for thiswork and quenching work was performed according toASTM D 6482. Samples after carbonitriding wer
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