INOX – Steel processing guide
The bonding techniques can be adopted with steels are varied, both in fixed joints and removable joints.
We begin to consider, as a typical example of fixed joints, the
- ELECTRODE WELDING – It is the most widely used system for thick components over 1.5 to 2 mm. The most widely used coatings are rutile (most bathrooms and smooth sliding cords with good aesthetic characteristics) and the basic (less sliding, with less smooth cords, but with good mechanical characteristics).
It is, of course, necessary to provide a filler material of the same type of the base material and the weld bead pickle mechanically (with steel brushes or inert material) or chemically (with pastes).
- TIG WELDING – With inert gas protection (literally “Tungsten Inert Gas”), allows excellent welding with mechanical and aesthetic features, both on thin and major thicknesses.
The arc and the bath area are protected with a suitable inert gas (argon and hydrogen), therefore there is no need, after the welding, of mechanical or chemical pickling.
The TIG procedure is widely used with stainless steels in both manual and automatic version. It can be welded without filler metal, by simple blending the flaps. In such case, it is considered as the upper limit thickness, about 1.5 mm. The range of thickness usually recommended for welds in TIG extends from 0.5 to 6 mm; up to a thickness of about 3 mm, the welding can be performed on the plates with straight edges, without crimping. For thicker layers, it is necessary to smooth the edges at V shape.
- MIG WELDING – Means “Metal Inert Gas” and, unlike the TIG, it is always performed with the filler metal, and the weld is always protected by inert gas to prevent the formation of oxides.
The range of weldable thickness is 0.5 mm to about 15 mm; for greater thicknesses may be more advantageous the submerged arc system.
Compared to TIG, MIG allows a higher deposit rate and is therefore recommended for fills and especially for the execution of cords to the corner.
- PLASMA WELDING – It is based on the use of ionized gas that, sent on the material, creates a break-through (hole) in correspondence of the plasma emitted by the torch vein (technique of the “keyhole”). The torch is then advanced and the hole closes immediately after the passage of the plasma vein; this due to the surface tension of the molten metal. All this takes place always under gaseous protection: usually a shielding gas that is made with mixtures of argon and hydrogen.
This process allows to achieve a very narrow cord with a limited thermally altered area, given that the heating of the material is very localized.
In addition to the excellent welding characteristics, it can have a very wide range of weldable thickness, indicatively from 3 to 10 mm, with straight edges, without the need to chamfer.
- SUBMERGED ARC WELDING – It is used to weld very thick parts; the bow area is “submerged” by a granular or powder flow that is dispensed from a hopper, so as to maintain the arc out of contact with air, protecting the weld pool from oxidation and from pollution. This welding technique allows to obtain high-quality joints with good speed of execution, especially if the joint is accessible from both sides and you can also operate on the reverse side. In these conditions, you can perform welding with thicknesses from 7 to 15 mm, with a pass to the right and one on the reverse side,with straight and combined edges.
For more significant thicknesses, up to 40÷50 mm, it is convenient to use a preparation of the edges with V shape for lower thicknesses and double-Y with a 5÷15 mm shoulder for higher thicknesses.
- LASER WELDING – This technique certainly provides considerable advantages from the point of view of the goodness of the welded joint and the speed of execution. In particular, they can obtain extremely narrow beads, with very limited thermally altered areas.
In addition, there is the absence of contact between the workpiece and the electrode, and the absence of the filler material. The advantages are many, especially when it is need to automate the welding and minimize the shrinkage or distortion of the joint.
By contrast, the laser sources for welding currently have certainly very high costs compared to other techniques such as TIG, MIG, plasma, etc. Only a production of high series, where you require qualitative characteristics and repetitiveness of the coupling particular, today justifies a laser joining technique.
This bonding system normally acts on thicknesses from 0.5 to 6 mm.
- RESISTANCE WELDING – The stainless steels are well suited to any type of resistance welding, especially those of the ferritic range, given the low value of specific resistivity of the material. The welding resistance adopted for stainless steels is due to four different techniques:
- welding by overlapping points;
- welding by overlapping pads;
- welding for overlapping rollers;
- butt welding for glitter.
echniques are the normal ones relating to carbon steels; for stainless there are just a few things to keep in mind; in welding for overlapping points, for example, it is necessary that
- the pressure on the electrodes is high;
- the welding current intensity is high;
- the welding time is limited.
As part of the fixed unions performed on stainless steels, it is also to mention the junction effected via
With this operation, also called “capillary brazing”, it is possible to realize joints with filler metals having lower melting ranges compared to the base metal. There are two types of brazing, the sweet and the strong.
- SWEET BRAZING – It is performed with fluxing alloys at temperatures below 400 °C. The most widely used alloy is 50% tin and 50% lead, used for example in the plumbing sector. In this case it is appropriate to use orthophosphoric acid as deoxidizer flow.
- STRONG BRAZING – Instead, is performed with alloys that have a melting range above 400 °C, with copper-based alloys, silver-copper, nickel-chromium. The flows are usually at the state of powders which can be mixed to create a paste-like element. These are constituted by borates, borax, boric acid.
For fixed unions, we can also mention the techniques that involve linkages such as rivets, nails, etc. For these joints, it is necessary to provide stainless steel elements for the connection or, in any case, of a material such as to have compatibility with the stainless steel to be joined (such as monel, etc.). This to avoid development of galvanic corrosion phenomena that can damage the less noble element.
SCREWS AND BOLTS
For all joints made with removable parts, should be taken into account, as mentioned for the riveting and nailing, galvanic compatibility of the elements.
Considering that screws are the most used way to create connections, it is to keep in mind that the union member has, generally, limited surface dimensions than elements to be connected; therefore, if parts to be joined are of stainless steel, the screw, and optionally the nut, they must always be of steel, to avoid galvanic corrosion.
If the connecting element is made with a different material, it is possible to prevent the triggering of galvanic corrosion, by interposing an inert material (like rubber, Teflon, etc.) between the two metals to interrupt the metallic continuity.
If the junction is in strongly aggressive environments, it is necessary, during the assembly, tighten very well screw and nut, so as to avoid the creation of zones of interstice or interspaces that could be seats for the priming of corrosive phenomena.
The stainless steel hardware is, for the most part, obtained by cold forming (rolling), in particular cases, however, it can also be obtained by joining lathed parts: it is advisable, in this case, the use of materials with improved machinability.
Stainless steels for bolts are identifiable with specific markings adapted to identify the group and the quality of the different materials.
For the scheme of this type of identification, it is referred to UNI 7323, Part 8a of 1980 (“bolts with special features – technical requirements – of stainless steel bolts corrosion resistant”). This standard defines a system of designation with a letter and three numbers, which represent the type of steel used and the mechanical characteristics of the hardware.
Symbology is shown in table 4.
Table 4 – Identification of stainless steel for bolts (UNI 7323 – Part 8 a), with the quality and the different strength classes.
The topic of fixed unions can be concluded presenting the bonding technique that provides for the adoption of structural adhesives with which can be achieved excellent results from the point of view of mechanical strength of the joint.
This system certainly has considerable advantages compared to conventional union systems:.
- ability to combine stainless steel with other less noble metals;
- possibility to maintain unchanged the surface appearance of the metal;
- sealing of the nip, to avoid interstitial corrosion;
- possibility to maintain the metallographic structure of the steel, without high thermal alterations for the junction.
On the other hand, however, this joining technique has some limitations:
- inability to make joints subjected to too high thermal loads;
- the need to provide adequate substrate preparation and appropriate setting times;
- the need to properly design the joint to the applied type of stress.
To obtain good performance characteristics of the bonded joint, you must be aware of the following points:
- surface preparation;
- oint design;
- type of adhesive.
You must create the right conditions for the adhesive to conveniently cover stainless steel surfaces. To do this, it is necessary to “activate” the metal surface or chemically (for example, with oxalic acid or sulfuric acid), or mechanically with inert abrasive (alumina or corundum).
After the preparation, it is necessary to apply the adhesive within a short time, to avoid the steel become passive again.
It is unthinkable that a joint, designed to be riveted or welded, can either be pasted. It is indeed necessary that a junction with adhesive is stressed essentially in pure shear or tensile, avoiding concentrated loads or stresses acting in the sense of favoring cracking or peeling.
Type of adhesive
The most used types of structural adhesives, mostly based on synthetic resins, are grouped in table 5.
As can be seen, they can be mono or bi-components and in different aggregation states: liquid, solid or mushy. Adhesives can also be constituted by a thermoplastic or thermosetting resin, depending if it loses consistency or if it hardens with the temperature.
Therefore, if in service will require a resistance to thermal loads, we will adopt thermosetting resins, while, in absence of high temperatures, we will use thermoplastics.
Table 5 also shows the bonding mechanism and the possible presence of solvents which allow to obtain adhesives in the form of liquids, thus more easily applicable.
Table 5 – Some types of structural adhesives for stainless steels.