If extensive machining is to be carried out the use of free-machining grades 303 or 416 should be considered, but consideration must also be given to the relatively low corrosion resistance, wearability, and formability of these grades.
Improved Machinability grades such as the "Ugima" range (eg Ugima 304 and Ugima 316) are now available - these offer better machinability than standard stainless steels but still retain the excellent corrosion resistance, weldability and formability of their standard grade equivalents.
Stainless steel grades can also be selected for ease of cold forming; Grade 302HQ (UNS S30430) is a low work hardening rate grade available in wire form specifically for the cold forming of fasteners such as bolts and screws. By contrast Grade 301 and 304 have a very high work hardening rate and can be supplied in a heavily cold worked condition suitable for the manufacture of springs; these require no hardening treatment after forming.
Components to be welded must be fabricated from a grade selected on that basis; to avoid problems associated with "sensitization" - caused by holding in the temperature range of about 450 to 850°C - it may be necessary to use a low carbon "L" grade or a stabilized grade such as Grade 321. In the case of all welding, it is essential that welding consumables are selected to match the grade being welded.
Design to Avoid Corrosion
When designing for stainless steel fabrication it is necessary to be aware of the factors which can cause premature corrosion failures. The principal problems are:
- General corrosion - a widespread wall thinning caused typically by exposure to strong reducing acids particularly at high concentration or temperature
- Pitting corrosion - related to chlorides, even in low concentrations and particularly at slightly elevated temperatures
- Crevice corrosion - also related to chlorides but made worse by small crevices in which liquid is trapped
- Intergranular corrosion due to prolonged heating in either welding or in an application in conjunction with incorrect grade selection
- Stress corrosion cracking due to applied tensile stress, again in conjunction with chlorides and raised temperature
- Galvanic corrosion due to proximity of metals widely spaced in the electrochemical series
- Contact corrosion due to contamination of the stainless steel by a material such as mild steel particles.
Often measures to prevent several of these problems are similar. Design of stainless steel components must be made to prevent the build-up of stagnant water, to encourage circulation of liquids, to discourage evaporation-concentration, and to keep stresses and temperatures as low as possible.
Good design alone is not sufficient to prevent problems; fabricators must also be aware of these problems and may need to modify their practices accordingly
Specific Design Points – To Retain Corrosion Resistance
Invert Structural Members
Avoid entrapment of moisture within members and within attachments. Stagnant liquid remnants are likely to concentrate and to lead to pitting corrosion.
Ensure Tanks and Pipes Drain Fully When Idle
Tanks and pipelines left with small residual fluid quantities also encourage pitting corrosion. The problem is made worse if the fluid is spread to a thin film.
Raise Tanks Off the Floor
Tank bottoms placed directly on concrete floors will create crevices; ideal sites for corrosion in the event of liquid spillage. Sealing the gap improves the position, but is subject to misapplication and deterioration. A drip skirt prevents liquid collecting beneath the tank while raising the tank on legs removes the crevice entirely.
Smooth, Rounded Corners Inside Tanks
Efficient maintenance cleaning of tanks is often important to remove built-up debris or stains; this reduces the likelihood of crevice corrosion under sediments and also may be important to retain hygienic conditions or prevent product contamination. All internal tank corners should if possible be well rounded and smooth. Welds should be in tank sides, not at corners. Welds should also be ground smooth (much easier if the weld is in the side, not corner) and full penetration or from both sides. All of these measures improve the fatigue resistance of the structure, as well as removing crevices.
Insulation or Lagging of Pipelines and Vessels
Thermal insulation of tanks and pipes should be free of chlorides. The insulation should be clad to totally prevent the entry of water, as pitting corrosion or stress corrosion cracking can occur in the warm, moist environment. The outside of a hot tank or pipe can be a highly corrosive environment because of the evaporation of liquid resulting in very high localized chloride contents. The outside may in fact be a more corrosive environment than the inside!
Incomplete Filling Problems
Vapour phases given off by some fluids can be quite corrosive; in these cases, if the tank cannot be filled completely the vapor space should be well ventilated to remove the vapor.
When dosing or making up a tank a highly corrosive chemical may be added. In these instances, it is important that the inlet is located away from side walls and in a moving liquid stream so that the addition is quickly diluted.
Reduce Splashing Within Tanks
A further problem may be caused by splashing during filling or mixing - splash drops on the inside tank walls will undergo evaporation and hence the concentration of the corrosive species. Splashing should, therefore, be avoided, perhaps by ensuring that the inlet pipe terminates beneath the liquid level or by running mixing propellers slowly and fully submerged.
Corrosion of all types proceeds more rapidly at higher temperatures. It is therefore important that immersion heaters in vessels are placed so as not to locally heat any section of the vessel wall, and processes should be run at the lowest constant temperature possible.
Avoid Settling in Pipes and Vessels
Crevice corrosion can occur beneath the debris that settles out of the stagnant or slow-moving liquid, and in some environments, low liquid velocity also permits marine organisms to grow on the steel, with a similar increase in crevice corrosion initiation. Designs should both maintain a reasonable flow rate (about 1 m/sec has been shown to substantially decrease the pitting rate in seawater) and result in total draining when the operation ceases. Dosing the fluid with a biocide may be a solution to the fouling problem, but chlorinated biocides such as hypochlorites can themselves be highly corrosive to stainless steels. Over-dosing must be avoided.
When joining pipe runs or using butt welding or socket weld fittings it is highly desirable to achieve full penetration welds. Incomplete weld penetration will result in a joint which appears good on the outside but has severe crevices at the root of the weld. Full penetration can be facilitated by the use of consumable inserts or by GTAW with hand-fed filler metal.
Crevices are readily created when supporting rings, attachment pads, etc are welded to stainless steel vessels. Intermittent weld runs may give adequate mechanical strength but only fully sealed welds give freedom from crevices.
Welding Mild Steel to Stainless Steel
Mixed-metal welding can be satisfactory, generally using an over-alloyed welding consumables such as Grade 309L, but caution must be exercised to prevent carbon migration into the part of the stainless steel exposed to the corrosive environment. Differential expansion rates can also lead to excessive stresses, particularly attack welds. Stainless steel attachment pads between a stainless steel vessel and its mild steel support can assist in reducing these problems.
Weld Repair to Avoid Crevices
If a leaking tank is patched this should be done in a manner that avoids creating further crevices, either inside or outside. For example, a butt-welded patch should be used and the weld dressed rather than using a lap welded patch.
Preparation for Welding
Minimum amounts of energy should always be put into stainless steel welds. The material should be carefully prepared and shaped before welding commences. The welder should avoid chill casting the first metal laid down and shrinkage cracks in the final weld pools.
De-scaling of Welds and Surface Cleaning
To achieve maximum corrosion resistance all weld scale should be removed and the surface ground polished and possibly buffed to the specified finish. Best resistance to corrosion is achieved when the steel surface is mirror smooth and totally free of scale or other contaminants. Pickling or passivating will assist in this process, and "pickling paste" is available to easily carry it out.
Avoid Sharp Machined Corners
Machining should avoid sharp internal corners at sites such as changes of shaft diameter and the corners of keyways, as these may act as crevices both by their own geometry and by the tendency during service for material to build up in these sites (and perhaps be difficult to cleanout). Radius of corners will also improve the fatigue resistance of any component subject to fluctuating stresses, as it does also for other materials.
Avoid Sleeving of Shafts
Fitting of sleeves to shafts creates possible crevice sites between the inner and outer components. A better solution from this point of view is to machine from a solid bar.