Stainless steels sensitizing

Sidhom, H., Amadou, T., Sahlaoui, H., and Braham, C. (2007) Quantitative evaluation of aged AISI 316L stainless steel sensitization to intergranular corrosion comparison between microstructural electrochemical and analytical methods. Metall. Mat. Trans. A, 38,1269-1280. 

Figure 8 Sensitivity of the new photothermal camera to small depth defects. lmage of an EDM notch of 1mm long, 100 pm width and 200 pm depth on ANSI 304 stainless steel with a bad surface condition (ground surface, "Vi 2 -6 ).

The sensitivity curves are plots of maximum achieved sensitivity as a function of thickness of the object for a given focal spot size and source to detector distance. The best attainable sensitivity in image intensifier systems is a function of tube voltage, current, scattered radiation and the screen gamma. As a first step, stainless steel plates with thicknesses ranging from 5 mm-30 mm in steps of 5 mm were chosen. These plates had a length of 950 mm and width of 280 mm. The plate is positioned very close and at the center to the LI. tube. The extraneous... 

Aqueous formaldehyde is corrosive to carbon steel, but formaldehyde in the vapor phase is not. AH parts of the manufacturing equipment exposed to hot formaldehyde solutions must be a corrosion-resistant alloy such as type-316 stainless steel. Theoretically, the reactor and upstream equipment can be carbon steel, but in practice alloys are required in this part of the plant to protect the sensitive silver catalyst from metal contamination. 

Manufacture, Evaluation, and Safety. The manufacture of shampoos is a relatively simple operation requiring a suitable stainless steel kettie with provisions for heating and cooling and equipped with appropriately sized mixers. Although shampoos are easily handled during preparation, precautions should be taken to not aerate the product. Cream shampoos are particulady sensitive to aeration and require more special care in their manufacture. 

Tubercles grow on nonstainless steels and some cast irons. Sensitized stainless steel and a few other alloys are rarely affected. Surfaces must... 

Certain conditions, ultimately dictated by economics, make the substitution of more resistant materials a wise choice. Stainless steels (not sensitized) of any grade or composition do not form tubercles in oxygenated water neither do brasses, cupronickels, titanium, or aluminum. However, each of these alloys may suffer other problems that would preclude their use in a specific environment. 

Dents in tubing can induce erosion failures, especially in soft metals such as copper and brass. Welding and improper heat treatment of stainless steel can lead to localized corrosion or cracking through a change in the microstructure, such as sensitization. Another form of defect is the inadvertent substitution of an improper material. 

Most defects can be detected using one or more appropriate nondestructive testing techniques. However, in the absence of routine nondestructive testing inspections, identification of defects in installed equipment is generally limited to those that can be observed visually. Defects such as high residual stresses, microstructural defects such as sensitized welds in stainless steel, and laminations will normally remain undetected. Defects that can be detected visually have the following features ... 

Figure 15.16 Schematic representation of sensitized stainless steel.

Stainless steel is susceptible to sensitization when it is heated to the range of 900 to 1550°F (480 to 850°C). Since any welding operation involving stainless steel will produce such temperatures in the metals being joined, it would appear that all welded stainless steel would sensitize. However, sensitization is a function of both time and temperature, occurring most rapidly at temperatures near 1250°F (675°C). Metals that cool rapidly through this temperature range will not sensitize. Consequently, thin metal sections, which cool rapidly, are less susceptible to sensitization than thick sections. 

Note that sensitization will not result in weld decay in all environments. Stainless steels may be used in environments that do not require the full corrosion resistance of the alloy. In these cases, weld decay may not occur even though sensitization has taken place. 

Locations. Weld decay may affect welded stainless steels that have normal carbon contents and are not specifically inhibited for sensitization. Weld decay affects only the immediate weld area. 

Critical factors. The critical factors governing weld decay include the use of a sensitized stainless steel and the exposure of this metal to an environment that is sufficiently aggressive to cause degradation in the sensitized region. 

Specify low carbon grades of stainless steel. Since sensitization results from the formation of chromium carbides, one approach is to sufficiently reduce the level of carbon in the alloy. Reduction of the carbon level to 0.03% or less has been shown to be effective in preventing sensitization. The low carbon grade of 304 is designated 304L 316 is 316L. Note the cautions below. 

Note that low carbon or stabilized grades of stainless steel do not possess intrinsically greater corrosion resistance than their unadjusted counterparts. Their sole value in typical cooling water systems results from their resistance to sensitization and potential weld decay that can result when the metals are welded. It is therefore not economically justifiable to specify low carbon or stabilized grades of stainless steel for typical cooling water system components that are not to be welded. 

Reheating and quenching sensitized stainless steel may not be practical in many cases. Note also that the quenching operation can induce substantial residual stresses and warpage. 

When stress-relief-annealing 300 series stainless steel components, care must be taken to avoid slow cooling through the sensitization range (see Weld Decay in this chapter). 

Galvanic corrosion may occur at stainless steel welds if sensitization has taken place or if welding has produced unfavorable dissimilar phases (see Chap. 15, Weld Defects, particularly Case History 15.1). These forms of microstructural galvanic corrosion do not involve the joining of two different metals in the usual sense. 

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