Processing / Welding
Standard welding processes for this steel grade are:
- TIG-Welding
- MAG-Welding Solid Wire
- Arc Welding (E)
- Laser Beam Welding
Preheating is not necessary for this steel. Interpass temperature should not exceed 150°C. Heat treatment after welding is normally not usual. Austenitic steels have only 30% of the thermal conductivity of non-alloyed steels. Their fusion point is lower than that of non-alloyed steels, therefore austenitic steels have to be welded with lower heat input than non-alloyed steels. To avoid overheating or burn-through of thinner sheets, higher welding speeds have to be applied. Copper back-up plates for faster heat rejection are functional, whereas, to avoid cracks in the solder metal, it is not allowed to surface-fuse the copper back-up plate. This steel has an extensively higher coefficient of thermal expansion as non-alloyed steels. In connection with a worse thermal conductivity, a greater distortion has to be applied. When welding 1.4828 all procedures, which work against this distortion (e.g. back-step sequence welding, welding alternately on opposite sides with double-V butt weld, assignment of two welders when the components are accordingly large) have to be respected notably. For product thicknesses over 12mm the double-V butt weld has to be preferred instead of a single-V butt weld. The included angle should be 60 - 70°, when using MIG-welding about 50° is enough. An accumulation of weld seams should be avoided. tack welds have to be affixed with relatively shorter distances from each other (significantly shorter than these of non-alloyed steels), in order to prevent strong deformation, shrinking or flaking tack welds. The tacks should be subsequently grinded or at least be free fro crater cracks. 1.4828 in connection with austenitic weld metal and too high heat input the addiction to form heat cracks exists. The addiction to heat cracks can be confined, if the weld metal features a lower content of ferrite (delta ferrite). Contents of ferrite up to 10% have a favourable effect and do not affect the corrosion resistance generally. The thinnest layer as possible has to be welded (stringer bead technique), because a higher cooling speed decreases the addiction to hot cracks. A preferably fast cooling has to be aspired to while welded as well, to avoid the vulnerability to intergranular corrosion and embrittlement. 1.4828 is very suitable for laser beam welding. With a welding groove width smaller 0.3mm respectively, 0.1mm product thicknesses the use of filler metals is not necessary. With larger welding grooves a similar filler metal can be used. With avoiding oxidation within the seam surface during laser beam welding by applicable backhand welding, e.g. Helium as inert gas, the welding seam is as corrosion resistant as the base metal. A hot crack hazard for the welding seam does not exist, when choosing an applicable process. 1.4828 is also suitable for laser beam fusion cutting with nitrogen or flame cutting with oxygen. The cut edges only have small heat affected zones and are generally fre of micro cracks and thus are well formable. While choosing an applicable process the fusion cut edges can be converted directly. Especially, they can be welded without any further preparation. While processing only stainless tools like steel brushes, pneumatic picks and so on are allowed, in order to not endanger the passivation. It should be neglected to mark within the welding seam zone with oleaginous bolts or temperature indicating crayons. The high corrosion resistance of this stainless steel is based on the formation of a homogeneous, compact passive layer on the surface. Annealing colours, scales, slag residues, tramp iron, spatters and such like have to be removed, in order to not destroy the passive layer. For cleaning the surface the process brushing, grinding, pickling or blasting (iron-free silica sand or glass spheres) can be applied. For brushing only stainless steel brushes can be used. Pickling of the previously brushed seam area is carried out by dipping and spraying, however, often pickling pastes or solutions are used. After pickling, a careful flush with water must be done.
Remark
In quenched condition the material can be slightly magnetizable. With increasing cold forming the magnetizability increases. Heat resisting tubes are delivered regarding testing in accordance to DIN EN 10216-5 respectively DIN EN 10217-7. In Germany, SEW 470 still applies for heat resisting tubes.
