Gas Quenching Small Components

TRANSACTIONS OF MATERIALS AND HEAT TREATMENTPROCEEDINGS OF THE 14IFHTSE CONGRESSOctober 2004Gas Quenching Small ComponentsPaul StrattonBOC, Sheffield, UKAbstract: Gas quenching is an environmentally friendly and effective way of quenching components. However it has noteen possible to apply it to small components because of the difficulties of jigging. A method of gas quenching such smallcomponents is described in which they are levitated in a confined gas stream. The method is suitable for quenching lowKey words: gas quenching, levitation, small componentsIT HAS BEEN SHOWN thathing is aenvironmentally friendly quenching method that can be 1. The Quench Chamberapplied to bulk loads in vacuum [1] or conventionalFor "light"components a quench chamber can befurnaces [2] and to individual components[31. It gives conceived that requires only the lower nozzle field.Amore uniform results and hence less distortion than nozzle field (or equivalents such as a high wire to holeconventional liquid quenchants [4]. Until now, therehas been no way to apply this technology toratio wire mesh above a plenum chamber or acomponents that are too small to be effectively Jgged perforated plate like that used in fluidised bed furnaces[6 )produces an almost uniform high velocity gasto keep them separate during the quenching processstream in an enclosed chamber. The"nozzle"field is atxperiments have shown that quite large and heavy(O 75kg) components can be floated in a nozzle field the bottom, the sides are solid and the top open(Figure1). The sides of the chamber are angled slightly[5]. In this technology an upper nozzle field exerts a outwards to produce a constantly falling gas velocity inforce equal to that from the lower field minus thecomponent weight and keeps it in place. This is order to give more control over the components beingobviously not practical for small, light components and quenched. It has been shown in other applications thatanother method of constraining them is requiredthe maximum divergence angle is 7o if a uniformvelocity field is to be maThe volume of gas usedThe slope of the wallsst be less than thatessentially uniform flowThe volume of gas usedrequired velocity herePerforated plawire mesh. The areaof the holes must beless than the area ofThe parallel portion of thethe inlet pipe to thevessel must be long enoughplenum chambeto achieve essentially uniformFigure 1. A schematic of the quenching chamber(not to scale)中国煤化工CNMHGTRANSACTIONS OF MATERIALS AND HEAT TREATMENTComponents enter at the top with the gas switched velocities and the larger the difference requiredon to prevent them falling to the bottom Non-spherical between top and bottom diameters of the quenchcomponents will each offer the gas stream a different chamber. The chamber design must therefore take intoprojected area and hence receive a different lift. As account the largest aspect ratio component to belong as the velocity of the gas field at the point where it processed almost regardless of its size. Size and theis effectively uniform is greater than the velocity number of components to be processed simultaneouslyrequired to"lift"the component when it has its smallest only need to be taken into account when consideringprojected area, the component will be levitated by the the bottom diameter size. The quench chamber must begas field and be quenched while it floats. Thedesigned slightly deeper than required byinevitable inconsistencies in the gas field and calculations to allow for the momentum duringturbulence will make the components tumble over and tumbling adding to both upward and downward motion.over so that they are uniformly quenched. The slopingIt is somewhat counter-intuitive to find that usingwalls and reducing gas velocity will stop components this technique large heavy objects with large aspectbeing ejected. The tumbling should give the ratios have a higher heat transfer coefficient than lightcomponents a Newtonian distribution of height, but spherical objects or even light, high aspect ratio objectssome ejection is inevitable unless the walls are slopedHowever, the heat transfer coefficient is not the onlyfactor that needs to be considered. The absolute2. Quencheramount of heat to be removed also affects the coolingThe first example is a small cylinder manufactured rate. Thus for a given heat transfer coefficient, smallerfrom steel(density 7840 kg/m3). It is 3 mm inomponents cool fasterdiameter and 30 mm long and therefore weighs for all shapes. The results for some typical shapes are0.001568 kg. The quenching gas is nitrogenThe general formula for the gas velocity required toshown below in Figures 2 and 3. The heat transfercoefficient used is an estimated average based on alevitate the component is:combination of different gas flow conditions duringV=(2*M*9.81)(A*D)tumbling. The calculated cooling rates are forconvective heat transfer only and do not include theffects of radiation heat loss, which are relatively smallwhere V is the gas velocity in m/s,at600°C.M is the mass of the component in kg,a is the projected area of the component in mand d is the density of the gas in kg/mFor this component the minimum projected area is0. m. The density of nitrogen is 1. 1605 kg/m50at 21. 1C and 1 atmosphere pressure. The maximumrequired gas velocity is therefore 62 m/s. Thecomponent's maximum projected area is 0.00009 mThe minimum required velocity is therefore 17. 2 m/sTo achieve this velocity difference, the area of thequench chamber must increase 3.6 times between theg20maximum and minimum velocity points to preventcomponents hitting the bottom of the chamber or beingejected at the top. This is equivalent to a 1.9 timesincrease in the diameter of a circular cross section0chamber. At the maximum design slope, this implies10that the distance between the upper and lower velocitypoints is 3.66 times the diameter of the bottom of theDiameter(mmchamberCalculation suggests that the heat transfer coefficientfor this component will be of the order of 120 to 220transfer coefficient will be in the range 300 to 330 levitated in nitrogen te of steel spheres at 600oC' mC. For a cylinder ten times the size, the heat Figure 2. CoolingW/m"rc because the gas velocity needed to levitate itLow alloy steels, e. g. SAE 5115, will fully harden ingas with a cooling rate of around 20C/s or higherAs might be expected, the larger the aspect ratio, the Thus for eramnle from Fionre 2, spheres of low alloylarger the difference in minimummaximum中国煤化工 diameter is less thanCNMHGPROCEEDINGS OF THE 14 IFHTSE CONGRESSOctober 2004approximately 3mm. Other materials have lower complex jigging. If higher hardenability materials arecritical cooling rates. For a material with a critical used then much heavier components can be processedate of 15C/s, e.g. 25MoCr4, as shown in The process is particularly suitable for larger complexFigure 3, cylinders weighing Ig or less will be fully components of high hardenability where evenquenching is required, especially as there is no need fora jig to hold components in position during quenchingultiple components may be quMultiple components must be quenched as a batch orsemi-batch process where additional components may\ f o be added to those quenching but no components can beremoved until the last components to enter have been0.5kgquenched. The number of components that can be二。.1krocethey should be able to tumble freely, which will varyonsiderably depending on their geometry3.1 Removing the Components20Several techniques are available to remove theAspect ratiocomponents from the device at the end of the quenchcycle.. In the first a door is fitted into the side of thechamber near the bottom. The gas velocity is reducedFigure 3. The effect of aspect ratio on the cooling rate slowly to zero to prevent severe impact of thecomponents on the bottom of the chamber. The door isof steel cy linders at 600@C for constant weights during then opened and the components are removed. For thislevitation in nitrogenmethod the components must not be so small that they3. Applicationswould fall through the holes in the base of the chamberAs an alternative the plenum chamber can be made toThe components would normally be discharged into be detachablethe device from a batch furnace typically used forAnother alternative is to temporarily increase the gastreating small components, such as a rotary barrel typeow so that its velocity at the top exceeds the velocityHowever, they could be fed in from a semi-continuous needed to levitate the components. The componentsfurnace such as a rotary scroll where a door discharges will then be ejected from the chamber and may becomponents at each revolution, provided there was time captured by a suitable device such as a bag house orbetween revolutions to quench and discharge thehe vortex separator. This method has the advantage thatcomponents. Components could also be transferred there is no risk of small components Falling througheither mechanically or by hand from almost any other the floor of the chamber, but it does increase runningbatch or semi-continuous furnace. A continuous costs.furnace would require the use of two chambers, oneA last possibility is to create a door above the levelquenching and one discharging at any given time. Each of the base, which is opened while simultaneouslywould need to have sufficient capacity to hold the restricting the flow at the top. The components aredischarge of the furnace for a period equal to the then carried through the door by the diverted gas flowquenching time plus discharge time. The exact method to a capture device. This method requires less gas butand means of coupling would vary from furnace to is mechanically more complexfurnace3.2 GasesIt is unlikely that the gas used by the quench could Throughout this paper nitrogen has been given as thebe reused by feeding it back into the furnace example gas, as it is the cheapest in most locationstmosphere as the top of the quench vessel must be Other gases such as argon, helium, hydrogen andopen or semi-closed with a vent, whose area must not carbon dioxide or mixtures of these gases could also bebe less than the area of the bottom of the chamber. The used. A mixture containing nitrogen with a smallintermittent nature of the quench cycle would also hydrogen addition(typically 2% but in any case lessthan that necessary to create a flammableIt is obvious that for lower hardenability materials would be preferred as this would resultmibrugetthe process is restricted to lighter components. The componentsrocess is particularly applicable to these lightcomponents as it produces even quenching in anenvironmentally friendly medium without the need for中国煤化工CNMHGVol 25 NosTRANSACTIONS OF MATERIALS AND HEAT TREATMENT4. Conclusions2. Holm T and Segerberg S,Gas seuengi branehes out149,No.6,With the correct design of quenching vessel it isossible to fully harden small, low alloy steel 3. Pp.64W-64Zcomponents with nitrogen as the quenchantroceedings of the 3rd International Conference on CFD inCalculation suggests a weight range of 1 to 10gthe Minerals and Process Industries CD-ROM (CSIRO,depending upon their aspectLarger componentsDevenny, D, 'Multiflow pressure quenching for distortion-can be fully hardened if they are manufactured from aree hardening, Metals and Materials, 1990, Vol 6, No. 2higher hardenability steel5. Stratton P F and Haring N, " Gas quenching carburisedlings of the 1" ASM InternationalReferencesSurface Engineering and the13th IFHTSE Congress CD-ROM(ASM Intermational, USA), 2002, Pp 17-2ller T, Gebeshuber A, V Strobl and G Reithofer, "New 6. Reynoldson R W, "Thermochemical surface treatments inibilities for Vacuum hardening", Heat Treating Progress,fluidised bed furmaces" Heat Treatment of Metals, 1980.Vol, No, pp 73-79Vol7,No.2,pp.35-40.中国煤化工CNMHG