Knife-line attack

A somewhat similar phenomenon is knife-line attack which may be observed after welding titanium or niobium stabilised austenitic stainless steels. In this case there is a very narrow band of severe intergranular attack along the interface between the parent metal and the fusion zone. During welding, the parent metal immediately adjacent to the fusion zone is heated to just below the melting point and both chromium carbides and niobium or titanium carbides dissolve completely. On cooling rapidly, the conditions are such that when relatively thin sections are welded, neither chromium carbide nor niobium or titanium carbide have time to precipitate. If the weld is now... 

Henthorne , in considering the corrosion testing of weldments, points out that the test will also give high rates due to (a) end-grain attack, which is particularly prevalent in resulphurised or heavily cold-worked material and (d) dissolution of Ti(C, N) such as occurs in Type S2J weldments and leads to knife-line attack. Since most service conditions do not cause attack on the alloy in these conditions the test can be misleading. 

Knife-line Attack severe highly localised attack (resembling a sharp cut into the metal) extending only a few grains away from the fusion line of a weld in a stabilised austenitic stainless steel, which occurs when the metal comes into contact with hot nitric acid and is due to the precipitation of chromium carbides. 

The selective corrosion of cast iron (graphitization), the preferential corrosion of the steel welding (grooving corrosion), sensitization and knife line attack of welded stainless steels are typical examples of corrosion influenced by metallurgical parameters. 

Corrosion resistance of aluminum alloys. Researchers have shown that aluminum alloys, both welded and unwelded, have good resistance to uninhibited HN03 (both red and white) up to 50°C. Above this temperature, most aluminum alloys exhibit knife-line attack (a very thin region of corrosion) adjacent to the welds. In inhibited fuming HNO3 containing at least 0.1% hydrofluoric acid (HF), no knife-line attack was observed for any commercial aluminum alloy or weldment even at 70°C (160°F). 

V. Cihal, Metallurgical Aspects of Knife-Line Attack on Stabilized Stainless Steels, Localized Corrosion NACE 3, R.W. Staehle, B.F. 

The phenomenon of knife-line attack within weld HAZs describes susceptibility to ICC and IGSCC in stabilized grades of austenitic stainless steels [61, 68]. Stabilization is a term used to describe depletion of solid solution carbon due to niobium and titanium alloying. These elements produce carbides in the temperature range from 870 to 1150 °C in austenitic stainless steels such as AISI 347 [61]. Little carbon remains in solid solution to be precipitated as (Fe,Cr)23C6. Normally, the initial get-tering of carbon above 870 C eliminates sensitization by Cr-carbide formation that normally occurs over the range from 425 to 815 °C in austenitic stainless steels. 

Unfortunately this process can be thwarted in the case of knife-line attack. 

A special form of intergranular corrosion is knife-line attack, which can occur in Nb/Ti-stabilized austenitic stainless steels after incorrect heat treatment [7.1]. 

It has been su ested that the chromium carbide (Cr23C6) formed at the grain boundary is only in registry with one grain and not with other grains. The magnitude of the knife-line attack is found proportional to the magnitude of mis-orientation at the carbide austenite interface. 

On the basis of chromium depletion theory, a uniform attack occurs on both sides of the grain boundary carbide. Therefore, the observation that the knife-line attack (knife-line attack occurs in a very narrow band in the parent metal immediately adjacent to the weld), occurs only on one side of the carbide, austenite interface cannot be explained by the chromium depletion theory. The knife-line attack arises as a result of strains... 

This phenomenon is restricted to the stabilized grades of steel, such as 321 and 347. The knife-line attack occurs immediately adjacent to the weld and shows as a thin line of intergranular corrosion. It results from intergranular corrosion like the weld decay. It may be noted that weld decay develops at some distance away fi"om the weld. The following is the mechanism suggested for the knife-line attack of stabilized steels. 

Knife-line attack in stainless steels is caused by one of the following ... 

Why stainless steels which are stabilized undergo knife-line attack If the zone on which knife-line attack has occurred is reheated between 950-1450°C, would you expect the steel to be sensitive to intergranular attack ... 

Differentiate clearly between knife-line attack and weld decay in terms of mechanism. 

Knife-line attack, immediately adjacent to the weld metal, is a special form of sensitization in stabilized austenitic stainless steels. Stabilizing elements (notably Ti and Nb) are added to stainless steels to prevent intergranular corrosion by restricting the formation of Cr-rich grain boundary precipitates. Basically, these elements form carbides in preference to Cr in the austenitic alloys. However, at the high temperatures experienced immediately adjacent to the weld fusion zone, the stabilizer carbides dissolve and remain in solution during the subsequent rapid... 

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