CLAUSULAS Y CONDICIONES DE LA OFERTA

The duplex grades are readily weldable with a filler metal of similar but enhanced composition. Although modern grades are of low carbon content, corrosion problems may arise from conversion of ferrite to sigma phase in the heat-affected zone.

V. POSTWELD HEAT TREATMENT

Postweld heat treatment (PWHT) consists of solution annealing and tem- pering or stress relieving, whichever is required.

The martensitic grades of stainless steel will have been welded in the annealed condition because in the hardened condition the hard brittle mar- tensitic structure does not have sufficient ductility to withstand the stress created by the thermal change. Prior to welding they must be fully annealed; otherwise severe weld cracking will occur. Having been preheated at 400– 600⬚F (205–316⬚C) and welded with a controlled interpass temperature, they are given PWHT high enough to form austenite and subsequently quenched to produce the hardened structure desired.

If less than 0.5-in. (6.35-mm) thick the ferrite grades of stainless need not be preheated. Thicker sections require preheating at approximately 300⬚F (150⬚C) to reduce shrinkage stresses and yield strength. Postweld heat treat- ment is used to transform any residual martensite to ferrite and to relieve stresses.

No preheating is required for the austenitic grades. The main concern is sensitization in the 800–1550⬚F (425–850⬚C) range. For the regular car- bon grades a PWHT consists of a solution anneal at about 1830⬚F (1000⬚C) followed by a water quench to avoid sensitization. This treatment is required only if the material is to be exposed in environments which are conducive to intergranular attack (IGA). For non-IGA exposures a thermal stress relief at about 1600–1750⬚F (870–950⬚C) for 2 hr/in. of thickness or 2 hr mini- mum will reduce residual stresses and prevent stress corrosion cracking. Low carbon or stabilized grades do not require a solution anneal. If SCC is po- tentially a problem, thermal stress relief as described above would be required.

The martensitic precipitation hardening grades are austenitic above 1830⬚C (1000⬚C) but undergo the martensitic transformation on cooling to room temperature. They harden by heat treatment in the 900–1100⬚F (480– 590⬚C) range due to the influence of such alloying elements as copper, mo- lybdenum, aluminum, titanium, and columbium.

The semiaustenitic PH grades do not undergo the martensitic transfor- mation. Postweld heat treatment at 1200–1600⬚F (650–815⬚C) causes pre-

cipitation of austenite-forming elements, permitting a martensite transfor- mation upon cooling to room temperature. They are then precipitation hardened at 800–1100⬚F (425–595⬚C).

The austenitic PH grades remain austenitic at all temperatures. They are annealed at 2000–2050⬚F (1090–1120⬚C) then hardened at 1200– 1400⬚F (650–760⬚C).

The duplex alloys may be solution annealed after welding if there is concern over sigma phase transformation in the heat-affected zone or to assure the desired ferrite/austenite balance and impact resistance.

VI. SOLDERING

Relatively few problems arise from temperature when soldering stainless steel. However, aggressive fluxes are necessary to prepare the surface for soldering. Because of this phosphoric acid type fluxes are recommended since they are not corrosive at room temperatures.

VII. BRAZING

All stainless steels can be brazed. However, since brazing alloys are usually composed of copper, silver, and zinc, substantially high temperatures are required. This can lead to such high temperature problems as carbide pre- cipitation and a reduction of the corrosion resistance.

VIII. PASSIVATION

The corrosion resistance of the stainless steels is the result of the passive oxide film which forms on the exposed surfaces. Under normal circum- stances this film will form immediately upon exposure to oxygen. Some fabrication processes can impede the formation of this film. To guarantee the formation of this protective layer, stainless steels are subjected to pas- sivation treatments.

The most common passivation treatments involve exposing the metal to an oxidizing acid. Nitric and nitric/hydrochloric acid mixtures find the widest usage. The nitric-hydrochloric acid mixtures are more aggressive and are used to remove the oxide scales formed during thermal treatment. This process provides two benefits. It removes the oxide scale and passivates the underlying metal. Second, the passivation process will remove any chro- mium-depleted layer that may have formed as a result of scale formation.

For passivation treatments other than for scale removal, less aggressive acid solutions are used. The purpose of these treatments is to remove any contaminents that may be on the component’s surface that could prevent the

formation of the oxide layer locally. The most common contaminent is em- bedded or free iron particles from forming or maching tools. A 10% nitric acid solution is effective in removing free iron. For martensitic, ferritic, and precipitation hardening grades a nitric acid solution inhibited with sodium dichromate is used so as not to attack the stainless steel too aggressively. A 1% phosphoric acid solution and 20% nitric acid solution are used for the more resistant stainless alloys.

IX. SANITIZING

When stainless steel is to be used in food service it requires treatment to remove bacteria or other microorganisms. It is quite common to use chlorine water or hypochlorite solution for this purpose. These solutions should be made up using demineralized water. This process can be successful provided that the solution is properly drained and flushed. A conductivity test may be used on the rinse water to ensure that the discharge is substantially equiv- alent to the demineralized water used in formulating the sanitizing solutions. If not thoroughly rinsed, chloride pitting, crevice corrosion, or stress cor- rosion cracking may occur.

Other safer alternative oxidizing solutions such as ammonium persul- fate, hydrogen peroxide, dilute peracetic acid, or a citric/nitrate solution should be considered. Another possible approach is the use of a nonoxidizing biocide such as hexamethylene biguanide or other environmentally safe bi- ocides. These are free of the hazards associated with chlorine, hypochlorite, chlorine dioxide, or other halogenated agents.

X. PREPARING FOR SERVICE

Once fabrication is complete and the material is ready to be placed in service it is essential that steps are taken to preserve the protective film of chromium oxide. The most common causes of problems are

Iron contamination Organic contamination Welding contamination

A. Iron Contamination

As mentioned previously embedded iron can be removed by pickling. This is primarily an operation required on fabricated vessels. However, care must be exercised in the storage and handling of stainless steel sheet or plate to prevent the surface from becoming contaminated with embedded iron. If cleanliness on the surface is extremely important, such as in pharmaceutical

or food environments where product contamination would be detrimental, the sheet or plate can be ordered with a protective adhesive paper on the surface. Leaving this paper in place during fabrication will reduce the amount of time required for cleanup after fabrication. The sheet and plate should be stored upright, not lain on the floor.

During fabrication it is good practice to use cardboard or plastic sheets on carbon steel layout and cutting tables, forming roll aprons, and rollout benches. This will go a long way in reducing or preventing iron embedment. The use of plastic, wood, or aluminum guards on slings, hooks, and the forks of forklift trucks will further reduce the chance of iron embedding.

B. Organic Contamination

Organic contamination is the result of grease, construction markings (crayon), oil, paint, adhesive tapes, sediment, and other sticky substances being allowed to remain on the stainless. If not removed, they may cause crevice corrosion if the stainless steel is exposed to extremely corrosive atmospheres. During fabrication there is little that can be done to prevent this contamination from occurring. The only solution is to insure that all such deposits are removed during final cleanup.

The cleanup procedures to be followed will depend somewhat on the service to which the vessel is to be put. In very corrosive media, a greater degree of cleanup will be required than in relatively mild media.

Good commercial practice will always include degreasing and removal of embedded iron. A complete specification for the procurement of a vessel should include the desired cleanup procedures to be followed, even if only degreasing and removal of embedded iron are required.

C. Welding Contamination

In corrosive environments corrosion will be initiated by surface imperfec- tions in stainless steel plate. This corrosion can occur in the presence of media to which stainless steel is normally resistant. Such imperfections can be caused by

Weld splatter

Welding slag from coated electrodes Arc strikes

Welding stop points Heat tint

Weld splatter produces small particles of metal that adhere to the surface, at which point the protective film is penetrated, forming minute crevices where the film has been weakened the most. If a splatter-prevention paste

is applied to either side of the joint to be welded, this problem will be eliminated. Splatter will then easily wash off with the paste during cleanup. Whenever coated electrodes are used, there will be some slag around the welded joints. This slag is somewhat difficult to remove, but if it is not done, the small crevices formed will be points of initiation of corrosion.

Arc strikes and weld stop points are more damaging to stainless steel than embedded iron because they occur in the area where the protective film has already been weakened by the heat of welding. Weld stop points create pinpoint defects in the metal, whereas arc strikes form crevicelike imper- fections in or adjacent to the heat-affected zone.

It is possible to avoid weld stop defects by employing extensions at the beginning and end of a weld (runout tabs) and by beginning just before each stop point and welding over each intermediate stop point.

An arc strike can be struck initially on a runout tab or on weld metal, provided that the filler metal will tolerate this. If the filler metal will not tolerate the striking of an arc, then the arc must be struck adjacent to it, in or near the heat-affected zone, when it is necessary to strike an arc between runout tabs.

Heat tint results in the weakening of the protective film beneath it and can be the result of the welding of intervals in a vessel or the welding of external attachments. The heat tint must be removed to prevent corrosion from taking place in the tinted area.

Welding contamination removal is best accomplished using abrasive discs and flapper wheels. Although grinding has been used, this procedure tends to overheat the surface, thereby reducing its corrosion resistance. Its use should be avoided.

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