Hightech by Gerster:
Heat treatment solutions. Boronising.
1 Hightech by Gerster: Boronising, the treatment process for extremely stressed parts.
Boronising is a thermochemical diffusion process. At treatment temperatures in the range of approx. 800 to 1000 °C the surface of a workpiece is enriched with boron; closed boride coatings are formed. The high degree of hardness and also the particular structure of the coating result in an extraordinarily good resistance to wear.
The properties of boride coatings The production process Very high degree of hardness: Boronising leads to a growth in volume iron based alloys 1600 to 2100 HV which corresponds to approx. 25 to 30% nickel based alloys up to 2800 HV of the coating thickness. Therefore a titanium up to 4000 HV corresponding undersize must be provid- Optimum adhesive strength ed for finished parts if necessary. The Little tendency to cold welding resulting rugosity amounts to approx. Expansion coefficient comparable 4 µm. Following boronising, honing or with iron materials grinding with CBN or diamond is possible. Good temperature resistance In order to avoid breakouts, edges Microstructure photograph of 42CrMo4, High heat hardness and corners must have a radius which is boronised and tempered, single phase at least the same size as the coating boride coating (Fe2B) approx. 100 µm, The coating thickness can be controlled thickness. In order to reduce the change hardness 1800 to 2000 HV 0,025. via the treatment time. It varies from in size and warpage behaviour to a 5 to 10 µm for components for which the minimum we recommend carrying out tendency to cold welding is to be reduced stress free annealing for demanding up to values of 300 µm for components components before the final processing. for which a pure abrasion strain is resent. Due to the high thermal durability The choice of material is matched to this. of the boride coating the components The higher the alloy content the lower can still be hardened or tempered the attainable coating thickness usually is. following boronising whereby a good load capacity at high surface loads is attained on the one hand and good strength of the components on the other. Case hardened steels can be carburised, boronised and subsequently hardened for a better supporting effect of the boride coating. Along with the resultant high stability of the coating the tough proper- ties of the core are maintained.
2 Small parts made of case hardened steel, quenched and tempered steel and hot work tool steel, boronised in bulk blasting process, subsequently hardened or quenched and tempered.
3 Hightech by Gerster: Typical Applications.
The so-called powder pack method in The boride coating maintains the high which the components are exposed level of hardness without noticeable drop to boron containing granules is technically for short term loads up to temperatures well-engineered and economically suc- above 1000 °C. For this reason boronised cessful. Single parts and mass-produced tools are used for the hot forming of parts thus allow themselves to be treated metals or glass. in a safe process in an organised position Boronising is often used in construction or as bulk material. Partial boronising of textile machines for thread guides. is also possible. Gerster also provides this Due to the very high speeds a high abra- process in combination with classical sion load occurs on the contact surfaces hardening processes such as vacuum hard- of the yarn with the metallic parts. Often ening, hardening under inert gas these problems cannot be solved with conditions, case hardening, tempering, either classical hardening processes or partial hardening and surface hardening. with known coating processes although Mass produced parts which are exposed these protective coatings have an even to high stresses in a metallic joint are pro- greater hardness than boride coatings. tected against wear by the boronising. Boronising also has widespread applica- The correspondent part is hardened in tions in the area of extrusion technology the normal way. The joint is lubricated and injection moulding technology, where possible. Due to the good sliding in instrument manufacture and in general properties the partner is protected against mechanical engineering and manufacture excess wear and there are good fail-safe of apparatus. The food and automotive properties in the event of unintended industries also benefit with special uses of cracking of the lubrication film. Such the durable boronised components. applications are typical for conveying and transport equipment. Material: 42CrMo4 Boronised: 30 to 40 µm Tempered: 1000 to 1150 N/mm2 With highly stressed gears the lifetime is increased considerably with the boronisation of the gearing.
Wear Properties Protection against corrosion Comparative wear tests were carried out Corrosion resistance for iron based by means of grinding disc processes. materials in acidic environments is In this a boronised sample of 42CrMo4 improved. Alkaline stresses do was compared with a nitrided sample however result in a reduction of made of the same material. In this the corrosion resistance. linear wear erosion of the boronised sample was around 1000 times smaller than that for the nitrided sample.
4 Textile machine construction: Rotor cup for making threads. Quenched and tempered steel, partially boronised, quenched, tempered and coated.
Comparison of coating thickness/hardness
Coating thickness [mm]
1,000
0,100
0,010
0,001
0,0001 0 1000 2000 3000 4000
Hardness [HV]
Case hardening Boronising complements other heat treatment Nitriding processes with regard to hardness and normal Boronising Iron based alloys Boronising Nickel based alloys coating sizes. The possible combination with Boronising Hard metals case hardening, through hardening and surface Boronising Ti-alloys Thin layer technology (PVD/CVD) hardening leads to good load capacity.
5 Hightech by Gerster: The Materials.
Almost all iron materials from machine, Nickel based materials such as e.g. vehicle and apparatus construction inconel, hastelloy or nimonic have excel- are suitable for boronising. High alloy lent corrosion resistance properties tool steels, hot working steels and and are often used at higher tempera- corrosion resistant steels, sinter metals, tures. Wear protection measures such grey iron and spheroidal iron can be as nitriding or case hardening can howev- boronised successfully as well. Alloys with er not be carried out with these materials. a high proportion of aluminium or silicon For precipitation hardenable alloys only should however be avoided if tempering hardness values below 550 HV are is necessary following boronising. They attained. give a soft intermediate coating directly Boronised surfaces have a much better below the boride coating. wear resistance up to high operating temperatures with a reduced adhesion resistance at the same time. For the aviation and chemical industries pure titanium or a/-alloys (e.g.TiAI6V4) are boronised. The coating thickness is 10 to 20 µm.
6 Food industry: Milling disc made of case hardened steel. Carburised, boronised, hardened.
Hardness comparison
Hardness [HV]
2000 Surface hardness 2060 HV
1800
1600
1400
1200
1000 Surface hardness 950 HV
800 Surface hardness 780 HV Surface hardness 800 HV
600 Core hardness 600 HV Core hardness 420 HV 400 Core hardness 280 HV 200 Distance from surface 0 Boronised and core hardened Nitrided Core hardened Hardened material: 1.2363 material: 1.8550 material: 1.5752 material: 1.2067
The hardness curves show the typical differences between the four heat treatment processes: boronising, nitriding, case hard- ening and hardening. Boronising is used when the other processes are not adequate from a wear perspective.
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Hightech by Gerster.
Surface hardening Through hardening/annealing Thermochemical diffusion methods Induction hardening Hardening under inert gas conditions Carburising Dual frequency hardening Vacuum hardening with Carbonitriding Impulse hardening pressurised gas quenching Case hardening Flame hardening Tempering Gas nitriding Non-destructive determination Annealing under inert gas conditions Oxinitriding of hardening depth Stress relief treatments Gas nitrocarburisation Cryogenic treatments down to –180 °C Pronox Laser technology Precipitation hardening of Plasma nitriding Laser hardening aluminium alloys Plasox Boronising Brazing Performance enhancing treatments Under vacuum conditions for stainless steels SolNit-A®, Under inert gas conditions SolNit-M®, HARD-INOX® 200 Inductive
With flame Consulting and additional services 8.2010
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