Higher Alloys

If low or intermediate-chromium steel pipe will not resist corrosion adequately in refinery streams it will be necessary to use aluminum-coatings or high-chromium ferritic (Type 400) or austenitic (Type 300) stainless steels. Do not try to justify anything more expensive. It is almost impossible. When trying to choose between these possibilities, look at the over-all picture. Do not even consider corrosion resistance. The difference is slight. As a matter of fact, the difference in cost may also be less than expected. 

Do not use ferritic stainless steels containing more than 16% chromium in the 750 F to 1,000 F temperature zone. They invariably embrittle, because of precipitation of a chromium-rich constituent. Even the 11 to 13% chromium alloy sometimes seems to embrittle in this way, for reasons not clear. Unfortunately, this is a common temperature zone for many refinery and chemical plant processes. These ferritic stainless steels should not be used above 1,000°F, either. They will embrittle for a somewhat different reason the formation of an iron-chromium intermetallic compound called sigma phase. 

Austenitic stainless steels resist oxidation by either oxygen-bearing or sulfur-bearing process streams. Austenitic stainless steel pipe is not foolproof simply because it has corrosion resistance. Many plant operators are too optimistic, feeling that tight temperature controls are unnecessary with austenitic stainless steels. If 

Relatively short-time tests indicate that austenitic stainless steels are ductile at all temperatures. This conclusion should not be considered valid for steels exposed to thousands of hours at process temperatures. Long exposures between, roughly, 800°F and 1,600°F precipitate chromium carbide and sigma phase in many stainless steels, causing a significant loss in atmospheric-temperature ductility. Ductility at higher temperatures is less affected. 

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Quaternary and Higher Alloys. Further additions to commercial aluminum alloys usually are made either to modify the metastable strengthening precipitates or to produce dispersoids. 

The SHE can be expensive when only one fluid reqmres a high-aUoy material. Since the heat-transfer plate contacts both fluids, it is required to be fabricated out of the higher alloy. SHEs can be fabricated out of any material that can be cold-worked and welded. 

Higher alloys. Some higher alloys pit to a greater degree than lower alloyed materials. Inconel 600 pits more than type. 304 in some salt solutions. Just adding alloy is not necessarily the answer or a sure preventative. 

For refinery units such as hydrocrackers in which the hydrogen partial pressure is much higher, e.g., above l,350psi and the operating temperatures are above 800°F, the 2 A Cr-1 Mo steel is eommonly used. The higher alloy... 

Strength Higher Alloy stronger than the base metal... 

If the desired catalyst is to consist of two or more catalytic metals after leaching or if a promoter metal is to be included, the precursor alloy becomes even more complicated with respect to phase diagrams. The approximate proportion of reactive metal (aluminum) in these ternary and higher alloys usually remains the same as for the binary metal system for the best results, although the different catalytic activities, leaching behavior and strengths of the various intermetallic phases need to be considered for each alloy system. 

Connections by welding are used if the materials applied can be welded safely, and only up to the pressure level of 700 bar. The usual range of materials comprises low-alloyed- and higher-alloyed ferritic steels as well as austenitic types. 

The material is a nitrogen alloyed duplex steel. The development has been continued to higher alloyed types (f.e. 1.4467 X3CrMnNiMoN2564)... 

Where not covered by TEMA Class R and material specified for both sides is the same, CA should be 0.75 times the sum of the CAs for each side up to a % in. (6 mm) maximum. Where not covered by TEMA and alloy requirements for two sides are different and a solid alloy tube sheet is used, use CA for higher alloy aide as total. 

Higher alloy content lm- 1 proves basic characteristics 1 of type 309. / able for welded construction. Jacketed high-temperature, high-preaaure reactors, oilrefining-still tubes... 

In many cases, a small decrease in the corrosion resistance of weldments is tolerable. When the environment is particularly severe for the alloy being used, the weld may be attacked preferentially. This condition can be exaggerated by the area effects of the more noble base metal compared to the small weld zone. An alternate approach for field welding is fo selecf a higher alloy welding consumable so fhe weld deposif is more noble than the base metal. Preferential attack can also occur in the heat-affected zone of the base metal. This is typical of weldments made in standard type 304 where the carbon content will lead to chromium carbide precipitation. Of course, this condition cannot be avoided by using a different filler mefal and the only remedy is a postweld anneal. 

In many milling applications, mill manufacturers offer a choice of steels for product-contact surfaces (such as mill liner), usually at least one low-alloy carbon steel, and higher-alloy stainless steels. The exact alloys vary significantly with mill type. Stainless steels are used in applications where corrosion may occur (many wet grinding operations, but also high-alkali or high-acid minerals), but are more expensive and have lower wear resistance. 

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