Process for partially insulating surfaces of metal work pieces

#The invention concerns a process for insulating portions of the surface area in the thermochemical treatment of metals by covering these portions.

#In the thermochemical treatment of metals, for example in the nitriding, boriding, siliciding or vanadiding of iron workpieces, titanium or other metals in practice there is frequently the need to exclude specific portions of the surfaces of such workpieces from this treatment, i.e. to insulate. In the mentioned thermochemical treatment there are generally formed hard, frequently also brittle, layers which are neither solderable nor weldable. Therefore, the insulation of specific portions of a workpiece has only the object to be able to subsequently solder or weld these portions which are still workable. Besides there can possibly be placed on the surfaces which are excluded from the thermochemical treatment other requirements in regard to slip behavior, errosion behavior, electrical or magnetic properties.

#There are known in the practice several processes for insulation of portions of the surface areas of workpieces in the thermochemicl treatment which, however, are only suited for specific processes or do not fulfill the requirements for complete insulation. Thus, for insulation in gas nitriding pastes are used which contain certain organic and metal compounds and are set up by dipping or brushing. For insulation in bonding in baths or powders there is possible, for example, an insulation only by a 0.1 to 0.25 mm thick pore-free galvanically established copper layer or by covering with a correspondingly thick copper sheet.

#Commercial steels of different compositions, as for example CK 15, 42 Cr Mo 4 or 60 W Cr V 7, and cast iron were borided in a known boriding powder (90% silicon carbide, 5% boron carbide, 5% potassium borofluoride) for 3 hours at 900.degree. C. Specific portions of these workpieces were coated by a tape which consisted of a cotton fabric which in its 100 m.mu. thick adhesive layer contained a mixture of 95% copper powder and 5% tin powder in a layer thickness of about 30 m.mu.. While the non coated areas had a hardness of 1700 to 2000 HV 1, (Vickers hardness, measured with a load of 1 kp) the coated areas only had a hardness of 95 to 200 HV 1, depending on the type of steel. Residues were not detectable on the workpieces.

#Boriding was carried out as in Example 1 but at higher temperature (1000.degree. C.) and for a longer time (7 hours). The tape and composition of the metal mixture were the same as in Example 1 but the thickness of the metal layer was 60 m.mu. and the thickness of the adhesive layer was 150 m.mu.. The hardness values obtained corresponded to those of Example 1.

#Workpieces of CK 15 steel were nitrided at 580.degree. C. in a salt bath containing 35 to 40% CNO.sup.-, 1 to 2% CN.sup.-. (The composition was made of 72 kg KCNO, 21.8 kg Na.sub.2 CO.sub.3, 6,2 kg K.sub.2 CO.sub.3 and 2.3 kg KCN).