TECHNICAL INFORMATION

Special print: WIREWORLD 4/92

J.-M. Rufer, Metallurgist

New hardening conceptfor small profilesand thin metal strips

~ 370-1320

31.039.2

•International Technical Magazine for the Wire and Cable Industries

Small parts therapyNew hardening conceptfor small profiles and thin metal strips

J-M. Rufer

Heat Treatment

Small parts therapyNew hardening conceptfor small profiles and thin metal strips

Pay-offfollowedby a washingunit and feedsystem leadinginto the austenitizingfumace

Small parts that are mass-pro-duced from profiles or stripsare having to me et rising re-quirements on quality.Morethan anything else they mustdisplay exactly the right me-chanical and metallurgicalproperties.

J.-M. Rufer*

Massparts are being pro-duced from small profilesand thin strips at an in-creasing rate of output.The high-performance

machine tools and modern blankingpresses used on these production linesplace accordingly high demands on thequality of the semi-finished feed stock.Here the key quality criteria are thematerial's mechan:jcal characteristics,metallurgical properties, geometry, sur-face finish and surface regularity.

For manufacturers of such semi-finished products it is also important, of

* J.-M. Rufer is head of the research and develop-ment department at a manufacturer of heat treat-ment plants in Biel, SwitzerJand.

course, to minimize production costsand to optimize manufacturing condi-tions not only with regard to the qualityrequirements but also in respect ofsafety and environmental regulations.This in turn means that the builders ofheat treatment plants have to offerhigh-performance equipment thatscores high marks for on-the-job con-venience and safety as weil as for tech-nology and economic efficiency.

Hardening of small profiles ina puII·through furnaceConventional pull-through furnaces

for the hardening of carbon steel aregenerally equipped with oil quenchingfacilities. Such facilities can certainly

Heat Treatment

X 1000

Quenchingunder protective Air cooling

gas

there is none of the usual pollutionnuisance.

This particular heat treatment line forsteel profiles can be used to treat bothunalloyed and stainless steels. Thegeometry of the cooling elements isquickly adapted to a new product orprofile.

To guarantee a high quality offinished product it is also important toselect the right protective gas. For sim-ple hardening a sufficiently pure pro-

Adaptable cooling systemAnpaBbares Kühlsystem

~280'C

StabilizationQuenching

under protectivegas'

1Il

"1 1\ \\ \\ \\ ,,,,,,,

".... ,.... ,....~

~900'C

Austenitising fumaceAustenitisierofen

Figure1:Exampleof a coolingsystem for graduai hardening (martempering) o(smallprofilesunder protective gas

metallic bright surface after hardening,without the need for any subsequentcleaning, washing or pickling. ln otherwords, it is possible for the product topass directly into the tempering fumaceaiter leaving the hardening fumace.

A further great advantage of a harde-ning fumace that works under protecti-ve gas is the absence of quenching oil.With no oil vapours in the fumace, theusefullife of the heat-resistant steel ele-ments is prolonged considerably and

Figure2: Heat treatment fumace line for the hardening and temperingof steelprofiles:a) Austenitizingfumace for a maximumtemperature of 1150°C,b) Washingunit preceding heat treatment, M)Coolingelement under protectiveatmosphere, c) Temperingfumace for a maximumtemperature of 650°C,d) Qilspraying system

produce good results, but they do havea number of drawbacks, for example:• A large quantity of quenching oilmust be kept in stock• Maintenance of pumps, filters, coo-lers, etc. is often expensive• Oil vapours and possible oil blee-ding inside the hardening fumace chan-nel can shorten the life of heat-resistantfumace elements considerably• Washing is always necessary beforetempering as otherwise the temperingfumace will be polluted and the semi-finished product oxidized during thetempering• Discolouration is often observed onprofiles made of certain steel gradeswhich require a high hardening tempe-rature or which have a surface thatoxidizes readily• Starting the strands and replacingbroken strands often poses difficulties• The risk of Cire on fumace lines con-taining oil quenching tanks must alwaysbe taken seriously.

Most of the above problems can besolved or eliminated for small sectionprofiles or wires by hardening underprotective gas. For large-section pro-files and for steels of low hardenability,oil quenching is still essential.

Thanks to modem fumace enginee-ring, diverse small profiles and wires ofunalloyed steel (e. g. CK75) of the typeswhich require high-speed cooling andare generally quenched in oil can nowa-days be hardened under protective gas.With this method of cooling it is alsopossible to adapt the hardening profileto the particular requirements. For ex-ample, the use of martempering and astabilization zone inserted between twocooling steps can make the use of saltor melted alloyed lead baths super-fluous.

ln the martempering process thetemperature is raised and held abovethe level of martensitic transformation(MS) long enough to permit the entireprofile to stabilize, after which it is 10-wered again to a point below the MSline in order to obtain a martensiticstructure.

Used on small profiles, martemperingcan decrease hardening distortions andreduce the thermal stresses which pro-duce distortions during subsequent ma-chining or forming.

Figure 1 shows an example of a coo-Iing curve applied in a hardening fuma-ce for small profiles. One of the mostimportant arguments in favour of a pull-through fumace line for hardeningsmall profiles under protective gas isthat it tums out a perfect, clean and

Heat Treatment

tective gas and perfect tightness of thefumace will provide good protectionagainst oxidation. There is no need touse gas with a carbon potential requi-ring regulators or other control devi-ces, since the passage time through thefumace is very short and the protectivegas serves only to prevent oxidationand decarburation of the product.

A hardening and lemperingfurnace in actionA pull-through fumace for hardening

and tempering various profiles in car-bon steel (CK75) or stainless steel un-der protective atmosphere has the fol-lowing operating features:

Ten strands can be treated simul-taneously. With an output capacity ofapproximately 130kg/h for a profilesection of 1.8 mm2

, the pull-throughspeed equals 17rn/min. During its waythrough the fumace the product passesthrough the following treatment phases:After the laminated product is unwoundand straightened, the residual lamina-ting oi! is removed by washing and ul-

Figure3: Overallviewof the heat treatmentlinefor the hardeningand temperingofsteelprofiles

trasonics. The austenitizing and harde-ning fumace is built for a maximumtempe rature of 1150°C so as to treatstainless steels as weIl. It is equippedwith a cooling element under protectivegas (in this case N2 + 5% H2) whichalso allows for a stabilization stage at atemperature above the level of marten-sitic transformation in order to reduce

distortions and to optimize the harde-ning structure. Once the product is har-dened, it can pass directly through thetempering fumace which featuresstrong convection to guarantee a preci-se and homogeneous tempering tempe-rature.

Before arriving at the take-up standwith a sophisticated winding and tra-versing system, the product is spray-coated with anticorrosive oil. The keytechnical data of the fumace are shownin the diagram and tables in Figure 2.

Closing remarksExperience with the cooling systems

described above prove that it is notonly desirable but also possible toadapt heat treatment processes to to-day's requirements on quality, userconvenience and environmental pro-tection. There are good prospects forfurther improvements in the future. Toachieve the set goals it is necessary toperfect the various construction tech-niques and the application of protectiveatmospheres.

WIRES, PROFILES, TUBES, SCREWS and MASS PRODUCTS

HEAT TREATMENT FURNACE-LINES for

•www.soloswiss.com

Automatic heattreatment line for SCREWS

and MASS PRODUCTSup to 1300 kg/h

HardeningCarburising

CarbonitridingTempering Washing

Supplementary «Technical Information» available

«The new 321 'Pratic System' furnace with conveyor belt»Technical Information SOLO 321/01.037.9

«Equipment for Hardening of Tools»E. Meinhard, Technical Information SOLO 1320/15.128.1

«The Present Practice of Nitriding and Nitrocarburising»E. Meinhard, Technical Information SOLO 1220/10.058.2

«Experience report concerning the heat-treatment installation type SOLO-202»Or. Tschierschke, Technical Information SOLO 202/7.068.5

«Flexible computer controlled heat treatment»F Cramme, Technical Information SOLO 202/955-10.118.7

«Influence of the mode of heat treatment on the quality and characteristics of semi-productslike wires, profiles and strips»J.-M. Rufer, Technical Information SOLO 370-1280/01.038.8

Continuous heat treatment furnace-lines under protective gas with integrated quenching,tempering furnaces, washing machines.Maximum capacity up to 1350 kg/h, electrically or gas heated.

«From the small furnace to the large heat-treatment plant»Automatic treatment lines for soldering, brazing, carburizing,carbonitriding, hardening, degreasing and temperingVertical retort fumaces with gas circulation for theheat-treatment of long partsBell fumaces with forced gas circulationShaHfumacesFumaces with forced air circulationConveyor belt fumacesAutomatic conveyor belt tempering fumacesHardening fumaces for bright hardening of air-hardeningchrome and high-speed steels under protective atmosphereHardening fumaces with built-in quench tanksGenerator for endothermic and exothermic atmospheresAmmonia dissociatorsCar-bottom fumaces

Large chamber fumacesDouble cham ber fumacesHigh-temperature cham ber fumacesFumaces for solution annealing and precipitationhardening of aluminium alloysSalt bath fumacesPreheating fumaces for hot-pressingSintering fumacesSmelting fumaces with metal or graphite crucibleCupellation and incineration fumacesTemperature control panelsWashing and degreasing unitsLaboratory muffle fumacesLaboratory heating cabinetsElectronic tacking machines