Accelerated cooling for sinter-hardening

The success of sinter-hardening depends on the cooling rates achieved in the forced-convection zone. Accelerated cooling rates can provide the opportunity to reduce raw material costs by using leaner alloy steels, in addition to increasing production rates.

Benefits to our customers

•Reductioninpowderrawmaterialcost (cheaper alloys)

•Increasedflexibilityinmanufacturing

•Reductionofdimensionaldistortions– Due to reduced hardenability – As compared to heat treated parts

•Potentialforshorterfurnacefootprint – Belt savings

– Less maintenance – Space savings

•Increasedthroughput(stacking)

•Enableslargerpartstobesinter- hardened instead of heat treated

Sinter-hardening is a well established commercial technology combining sintering and forced-convection gas quenching of martensitic steels in one continuously processing furnace. It is valued for its cleanliness and process integration that, eventually, translate into reduced operating and capital costs. However, the quenching rates of the atmospheric pressure forced-convection cooling systems used in typical sintering furnaces are limited if compared to the conventional oil bath quenching, and this creates an issue when considering sinter-hardening of lower-cost, lean-alloyed, powder metallurgy (PM) steels. Consequently, new and more effective convection cooling systems are required for maximizing martensitic transformation during quenching by increasing the cooling rate.

Figure 1: Forced Convection Is the Key Tool for Increasing Quenching Rate of Sintered Steel Parts

1200

1100

1000

900

800

700

600

500

400

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100

0.0 10.0 20.0

1 – Oil Bath 2–LINBath 3 – Multi-jet 4–LINjet

30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.00

CR is a function of heat transfer coefficient and ∆T between the cooled part and the cooling medium

CR~dQ/dt=hA∆T/dtwhere: CR – cooling rate, K/s; Q – heat; J, t – time, s; h – heat transfer coeff., J/(m2 K); A – surface area, m2, and T – temperature, K,

ForCR2>CR1,thehardenabilityofsteelcanbereduced;i.e.,theextentofsteelalloyingcanbeless.Hence,CR2mayallowforacompletemartensiticquenchingoflean-alloyed,lesscostlysteels.

Delubrication SinteringForced

convection quenching

CRiscalculatedfrom embedded thermocouple readings

650C — >150C 600C — >200C 550C — >250C

dT/dt = CR

CR1

CR2

Slow cooling

PM parts

Time, minutes

Tem

pera

ture

, deg

. C

Deg

ree

Celc

ius p

er se

cond

Figure 2: Average Cooling Rates of 280g Gears during Quenching

17.0016.0015.0014.0013.0012.0011.0010.009.008.007.006.005.004.003.002.001.000.00

ThisillustratesthatthecoolingratesandtheLINjetquenchingmethodmatchthatoftheoilbathquenchingwithinthetemperaturerange important for the austenite-to-martensite transformation. The multi-jet mimics a convective cooling unit.

Shown in Figure 1, above, is the typical sinter-hardening temperature profile. If cooling rate, CR, is increased, less alloying (C, Mn, Mo, Ni, Cr, Si, Cu, Mn, …) is needed to obtain martensitic trans-formation, even though the critical cooling rate for martensitic transfor-mation of lean-alloyed steels is higher. So increasing CR is desired. Figure 1 shows the part temperature profile within a sinter-harden furnace.

Fundamentally, the CR increase is possible by making the coolant tem-perature lower, making the interface heat transfer coefficient higher, and increasing the contact area, A.

Figure 3: In-furnace Operation of the Direct LIN Jet Impingement Sprayers, View inside the Convection Cooling Unit, the L-type (Left) and S-type (Right) Sprayers are Positioned above the 36"-Wide (0.91 m) Belt.

Cooling sideCooling side

Belt direction

Sintering sideSintering side

LIN jets impinging on belt surface

Table 1: Apparent HRC Hardness Values Conventional Heat LIN Convective Treated1 Cooling Unit2 Cooling UnitCurrent Alloy 33 44 45Lean Alloy #1 24 40 44Lean Alloy #2 21 39 42

1 Oil quenched by external heat treating shop, external to PM facility. 2 Dimensional stability (ID and OD) of the LIN-cooled parts were within the same

tolerances as the current production.

Figure 4: Temperature Fields inside the Convection Cooling Unit (Varicool of Abbott Furnace Co.) Equipped with a LIN Jetting Sprayer at the Entry End, a Computational Fluid Dynamics (CFD) Prediction for One Set of Quenching Conditions.

Belt direction

Cooling sections

Convectivequenching

Liquidnitrogensprays

Transition

Towaterheatexchanger

Sintering

9.48e+028.96e+028.44e+027.92e+027.40e+026.88e+026.36e+025.84e+025.32e+024.80e+024.28e+023.76e+023.24e+022.72e+022.20e+021.68e+021.16e+026.40e+01

˚C

By using cryogenic nitrogen, we can increase ∆T by direct-jetting liquid nitrogen (LIN) onto the part surface. We can increase h, and even if the contact area A between the cryogenic jet and the part surface isn’t larger than conventional, the overall heat

transfer, dQ/dt, and CR will increase enough to transform lean-alloyed steels to martensite.

Industrial-scale cryogenic jet cooling tests were carried out using a large sinter-hardening furnace built by

Abbott Furnace Co. and equipped with a Varicool blower-driven convec-tive cooling unit. Manifolds with LIN spray-jetting nozzles were installed inside the Varicool on the entry side, not far from the end of the sintering zone, Figure 3. As planned, the jets expanding inside the furnace were reaching the belt surface without excessive evaporation.

The gears were loaded on a 36-inch- wide (0.91 m) belt of the furnace to the production capacity of about 500 lbs/hr (227 kg/hr). With the stainless steel mesh belt and the ceramic support plates for the sintered gears moving at 6 in/min (2.5 mm/s), the total mass of hot material entering the convective cooling unit was estimated to reach 1000 lbs/hr (454 kg/hr). Preliminary evaluations of the heat-and-mass bal-ances inside the cooling unit equipped with LIN jetting system, performed using Fluent CFD, Figure 4, have shown that sufficiently rapid quench-ing could be obtained when the LIN spray is directed toward the gears and the mesh belt, and carried out concur-rently with the normal operation of the Varicool unit recirculating N2-10 vol % H2 atmosphere across the water-cooled heat exchanger.

Resultant as-quenched hardness was comparably high (see Table 1 below), suggesting a complete or a nearly complete martenistic transformation. A starkly different transformation response for the convective cooling with and without LIN impingement was observed. Earlier industrial sinter-hardening tests, carried out in the same furnace and using the same loaded belt procedure, have shown that the average gear hardness mea-sured for the lean alloy parts was only 21–33 HRC, less than the 36–38 HRC threshold required for these industrial gear parts.

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Figure 5: Optical Microscopy of Sinter-hardened Gears Made of Lean Alloy 1 (Left) and Lean Alloy 2 (Right) After LIN Cooling.

100 µm 100 µm

ThispresentstypicalmicrostructuresofthegearsquenchedusingLIN cooling. The gears have a good degree of sinter, and their microstructures arepredominatelymartensitic.Etchedusing2%nital/4%picral.

To summarize, recorded cooling rates, apparent hardness values, and the steel microstructures show that the direct cryogenic impingement, ap-plied inside a concurrently operating conventional, blower-driven convec-tion cooling unit, can produce good quality lean-alloy parts, even with production-scale loading. The extent of the transformation was found to be excellent by the industrial norms and comparable to that of oil bath quenching, but without the thermal distortions associated with the latter. Industrial implementation of this new sinter-hardening method requires ret-rofitting or replacing existing convec-tion cooling units. Shorter belt furnace cooling zones can also be used with this method, and smaller, conventional sintering furnaces can be adopted for sinter-hardening operations.

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