Heat tinting

Figure 6.9 Irregular deposit and corrosion-product mounds containing concentrations of sulfate-reducing bacteria on the internal surface of a 316 stainless steel transfer line carrying a starch-clay mixture used to coat paper material. Attack only occurred along incompletely closed weld seams, with many perforations. Note the heat tint, partially obscured by the deposit mounds, along the circumferential weld.

Pigment Colour Heat Tinting Opacity Lightfastness ... 

We may also use the method of tint etching to produce color contrast in microstructure. Tint etchants, usually acidic, are able to deposit a thin (40-500 nm) film such as an oxide or sulfide on specimen surfaces. Tint etching require a very high-quality polished surface for best results. Tint etching can also be done by heat tinting, a process by which a specimen is heated to a relatively low temperature in air. As it warms, the polished surface is oxidized. The oxidation rate varies with the phase and chemical composition of the specimen. Thus, differences in the thickness of oxidation films on surfaces generate variations in color. Interference colors are obtained once the film reaches a certain thickness. Effectiveness of heat tinting depends on the material of specimens it is effective for alloy steels and other non-ferrous metals and carbides, but not for carbon or low alloy steels. 

Each specimen was single-cycle-loaded to cause a crack extension of between zero and 2 mm. The load-unload method was used, in conjunction with heat-tinting and fatiguing techniques [ ]. Crack extension (Aa) was measured at three locations across the thickness and averaged, while / calculations were based on the approximation [ ]... 

Heat tinting is another type of deposit etch. Exposing a polished sample (not in a plastic mount ) to elevated temperatures in air causes oxidation to occur. On the microscopic level, oxidation rates of microstructural constituents vary according to their reactivity in a given environment. 

Welding slag from coated electrode arc strikes Welding stop points Heat tint... 

Heat tint results in the weakening of the protective film beneath it and can be the result of the welding of internals 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. 

Specify good-quality welds and avoid heat tint scales (use inert gas back-up procedures, for example). Remove the heat tint scale mechanically (grinding, electropolishing and/or abrasive blasting) and/or chemically (pickling). 

Corrosion resistance can usually be maintained in the welded condition by balancing alloy compositions to inhibit certain precipitation reactions, by shielding molten and hot metal surfaces from reactive gases in the weld environment, by removing chromium-enriched oxides and chromium-depleted base metal from thermally discolored (heat-tinted) surfaces, and by choosing the proper welding parameters. 

Postweld cleaning is often specified to remove the heat-tinted metal formed during welding. Recent work has shown that cleaning by stainless steel wire brushing can lower the corrosion resistance of... 

This is a particnlar problem in applications in which the base metal has marginal corrosion resistance. The effect may be cansed by inadequate heat-tint removal, by the use of lower-alloy stainless steel brushes such as Type 410 or 304, or hy the redeposition of abraded metal or oxides. 

Effects of Heat-Tint Oxides on the Corrosion Resistance Of Austenitic Stainless Steels... 

The rate of oxidation for a stainless steel, and consequently the degree of depletion in the base metal, is independent of the alloy composition and is controlled by diffusion through the oxide. The oxidized, or heat-tinted, surface of a welded stainless steel consists of a heterogeneous oxide composed primarily of iron and chromium above a chromium-depleted layer of base metal. The properties of such a surface depend on ... 

The chemical composition of the base metal beneath the heat-tint oxide... 

The physical condition of the surface (contamination, roughness, thermomechanical history) prior to heat tinting... 

The adherence of the heat-tint oxide to the base metal. 

The defects, internal stresses, and composition of the heat-tint oxide make it a poor barrier to any corrosive media that might initiate localized corrosion in the chromium-depleted layer of base metal. 

The severity of localized corrosion at heat-tinted regions exposed to oxidizing chloride solutions is directly related to the temperature of the hot metal surface during welding. A heat-tint oxide on an austenitic stainless steel exposed to air first becomes obvious at approximately 400 °C (750 °F). As the surface temperature is increased, differently colored oxides develop that appear to be superimposed upon the oxides formed at lower temperatures (Table 12.6). 

Dark blue heat-tint oxides are the most susceptible to localized corrosion. It should be noted that gas-shielded surfaces do not form the same distinctly colored oxides as surfaces exposed to air during welding, but gas-shielded surfaces can also be susceptible to preferential corrosion. 

Whether a weld heat tint should be removed prior to service depends on the corrosion behavior of the given alloy when exposed to the particular environment in question. Preferential corrosion at heat-tinted regions is most likely to occur on an alloy that performs near the limit of its corrosion resistance in service, but certain solutions do not affect heat-tinted regions. 

Even when heat-tinted regions are suspected of being susceptible to accelerated corrosion in a particular environment, the following factors should be considered ... 

Table 12.6 Welding conditions and corrosion resistance of heat-tinted UNS 531726 stainless steel plate...

Heat input Welding current, A Centerline heat-tint color Maximum pit depth Number of pits on heat-tinted surface... 

Single-pass autogenous bead-on-plate GTA welds were made to heat tint the root surface of 6.4mm O/4 Inch) -thick plate samples... 

Duplicate coupons, each one with 25 x 51 mm (1x2 Inch) heat-tinted surface, were exposed to 10% FeCI solutions at 50 C (120 F). The weld face and edges of each coupon were covered with a protective coating... 

The cost and effectiveness of an operation intended to repair a heat-tinted stainless steel snrface. 

Grinding or wire brushing may not be snfficient to repair a heat-tinted region. Snch abrading operations may only smear the heat-tint oxide and embed the residual scale into the snrface, expose the chromium-depleted layer beneath the heat-tint oxide, and contaminate the snrface with ferrons particles that were picked up by the grinding wheel or wire brush. 

A stainless steel surface should never be abraded with a wheel or brnsh that has been nsed on a carbon or low-aUoy steel wire brushes with bristles that are not made of a stainless steel of similar composition should also be avoided. Conversely, attempting to repair a heat-tinted region with only a pickling paste or acid solution may stain or even corrode the base metal if the solntion is overly aggressive or is allowed to eontact the surface for an extended time. 

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