Knowledge Corner : January 2010
CONTROLS SPECIFIC TO STAINLESS STEELS
Stainless steel mean those steels which do not rust in air or corrode in contact with certain corrosive media. The ability of stainless steel to resist corrosion is due to high chromium content. Chromium forms a tight, hard, continous and highly, insoluable oxide film on the metal surface, which shields it from further attack by atmosphere or corrosive substances.
The stable, bright and shining surface of stainless steel is due to chromium. The chromium content varies between 11 and 27%. Corrosion resistance is usually proportional to the amount of chromium present: higher the chromium content, better is the corrosion resistance.
But welding can change the properties in the heat affected zone and so a careful procedure of welding must be applied.
CLEANING BEFORE WELDING:
The high chromium content of stainless steels promotes the formation of tenacious oxides that must be removed for good welding practice. The surfaces to be welded must be completely cleansed of all hydrocarbon and other contaminants such as cutting fluids, grease, oil etc. by suitable solvents. General preweld cleaning methods are 1) pickling with 10 to 20 percent nitric acid solution and 2) cleaning with stainless steel wire brushes that have not been used for any other purpose.
Carbon contamination can adversely affect the metallurgical characteristics, the corrosion resistance or both of stainless steel. Pickup of carbon from surface contaminants or due to contact with carbon steel materials must be prevented. Stainless steel shall not come in contact with unalloyed or low alloy steels. Fabrication of stainless steel shall be done separately in a work area and with tools only to be used for the fabrication of stainless steel.
Austenitic stainless steel are suscepitable to hot cracking caused by the high co-efficient of thermal expansion of the material in combination with the high affinity of nickel to the pickup of impurities like sulphur, forming low melting nickel sulphides. To avoid this problem a high level of cleanliness is required when welding austenitic stainless steel.
CLEANING AFTER WELDING
It is well known that stain less steel is rust and corrosion resistant only when its surface is in a polished condition. This resistance is supposed to be due to a continous thin film of chromium oxide formed on the surface. If this continous film is broken due to the presence of crevices, blowholes etc., attack by atmosphere or by chemicals can be as rapid as on mild steel.
Naturally, stainless steel weld joints should be finished by grinding and subsequent polishing, if they are expected to give maximum corrosion resistance. This finishing cooperation is an expensive proposition and the degree to which it is to be pursued should be determined by the nature of corrosive conditions to be met in service.
The first step is coarse grinding. The high co-efficient of thermal expansion combined with the low thermal conductivity of the austenitic stainless steel necessitates extreme care during grinding to prevent overheating with consequent wrinkling, distortion or heat discoloration (this latter occurs when the surface temperature rises above 3000c). With unstabilised grades, severe overheating also impairs the corrosion resistance. In consequence, the grinding wheel must never be allowed to dwell in any one spot and heavy pressures must be avoided.
When it is intended that this rough, grinding operation should be the final finishing operation, the surface should be acid cleaned and washed. This treatment will remove any metallic particles and surface contaminants which may have been forced into the stainless steel surface, and which if left might initiate corrosive attack.
The corrosion resistant characteristics of stainless steels may be adversely affected by the sensitization process occuring while welding stainless, in a certain temperature interval from about 600 to 9000c(from about 1100 to 16500) which promotes the precipitation (gathering) of chromium carbides at grain boundaries and the parallel loss of anticorrosive chromium from the base metal.
The above range of temperature occurs naturally not at the welding stainless location, where temperature is higher and lasts only for a short time, but in two strips of metal on both sides of the weld bead. This is the so called heat affected zone (haz) where the harmful effects take place.
In a sensitized joint the chromium, which is the main “stainless?ingredient, becomes sequestered or taken out of play and locally unavailable for the protective action. If not addressed correctly, welding stainless 18/8 steels may thus cause the loss of their protective property along sensitized paths. The welded material becomes prone to intergranualar attack in a corrosive environment.
Acid cleaning may be employed, as well as mechanical means. Stainless steel wool or brushes should always be employed for this purpose. Occasionally, acid cleaning of welded stainless steel piping systems after welding is required. Such acid cleaning is usually carried out with a solution containing 15% nitric acid. For highly oxidized surfaces, a solution of 15% nitric acid and 0.5 to 1.5% hydrofluric acid may be employed. Because this cleaning involves the use of acids, the precautions should be followed.
After heavy oxidation of the weld and heat affected zone, the corrosion resistance can be restored by pickling and passivation or by chemical cleaning followed by passivation.
- Welding Technology for Engineers, Edition 2008-Page 284
- Modern Arc Welding Technology, Edition 2008 ?Page 472, 473
- ANSI/AWS D10.4-86, Page 20, 23
- DEP 188.8.131.52 ?Gen ?October 1995, Page 25, 26.