High-chromium nitrogen containing castable alloy

Abstract

A corrosion and erosion resistant alloy comprising as mandatory elements besides iron, in % by weight, about 31 to about 48 chromium, about 0.01 to about 0.7 nitrogen, about 0.5 to about 30 manganese and about 0.3 to about 2.5 carbon. This abstract is neither intended to define the invention disclosed in this specification nor intended to limit the scope of the invention in any way.

Description

[0001] This application is a continuation-in-part of U.S. application Ser. No. 10 / 040,357 filed Jan. 9, 2002, the entire disclosure whereof is expressly incorporated by reference herein.[0002] 1. Field of the Invention[0003] This invention relates generally to the art of alloys and more particularly to a high-chromium, nitrogen containing alloy having high corrosion resistance. The instant invention also relates to a high-chromium, nitrogen containing castable alloy, a high-chromium nitrogen content alloy, and a process for producing the high-chromium, nitrogen containing alloy, and articles prepared from the same.[0004] 2. Discussion of Background Information[0005] Equipment used in highly corrosive environments typically is made of alloys such as stainless steel and other highly alloyed materials. These alloys must be able to withstand the extremely corrosive environments created by chemicals such as concentrated sulfuric acid or concentrated phosphoric acid. A particularly diffic...

AI Technical Summary

Benefits of technology

[0014] Nitrogen, like carbon, forms interstitial solids with body-centered-cubic (bcc) .alpha.-iron, and face-centered-cubic (fcc) .gamma.-iron. The size of the nitrogen atom is smaller than that of the carbon atom, wherefor the nitrogen atom can occupy the interstitial sites in the .alpha.-as well as in the .gamma.-phases more easily than the carbon atom.

[0015] The maximum solubility of nitrogen in Fe-.alpha. and Fe-.gamma. is several times higher than that of carbon at the same temperature, which leads to a substantial expansion and distortion of elementary lattices. Nitrogen has a solid solution hardening and strengthening effect that is much greater than that of carbon, while at the same time maintaining a higher level of toughness.

Problems solved by technology

A particularly difficult environment is encountered in the production of phosphate fertilizer.

The crude phosphoric acid produced can be extremely corrosive and contains some residual sulfuric acid.

A particularly corrosive environment is encountered during the concentration of the crude phosphoric acid.

The most severe corrosive environments are typically encountered in the processing of deposits of phosphate rock which contains a high content of halides, such as chloride or fluoride.

However, both alloys exhibit poor mechanical properties, especially low toughness, brittleness, sensitivity to heat, and low notched impact resistance, thereby limiting their usefulness.

The ferritic structure comprised in these alloys is inherently very brittle, and the carbide phase embedded in such a brittle phase results in a very low toughness, high notch sensitivity, as well as sensitivity to heat.

Additionally, the ferritic structure is supersaturated with chromium, resulting in a formation of the sigma phase, which drastically lowers toughness and corrosion resistance.

The alloy of the '801 patent possesses good toughness, but exhibits very poor hardness and very poor wear resistance and low tensile strength.

Its hardness of 208 to 354 HB is similar to that of CD4MCU stainless steel (260-350 HB), which has excellent corrosion resistance, but poor wear resistance.

The alloy disclosed in U.S. Pat. No. 5,320,801 is similar to austenitic, high-nickel stainless steels in that it has good toughness, but very low tensile strength and hardness, as well as poor wear resistance.

The nickel present in corrosion resistant alloys mainly assists in structural stabilization but contributes very little to an improvement in corrosion resistance.

This difficulty is due to the precipitation of the sigma phase from alloys which are saturated with chromium and molybdenum.

The main contributing factors in this respect are very low toughness, brittleness and low endurance.

Very often a failure occurs due to a casting which is worn thin in an isolated area where, due to the poor mechanical properties of the alloy, a crack developed.

Eventually, this leads to the destruction of an otherwise still viable component.

This is most evident in alloys which contain a higher content of Cr and Mo, where a significant amount of sigma phase is unavoidable and the metal matrix possesses very poor toughness properties.

An increase in the Cr / C, or (Cr+Mo) / C ratio increases the corrosion resistance up to the critical point, after which the formation of the sigma phase begins, which drastically reduces the toughness and lowers the corrosion resistance of the alloy by depleting the Cr in the vicinity of the sigma phase precipitates.

The maximum solubility of nitrogen in Fe-.alpha. and Fe-.gamma. is several times higher than that of carbon at the same temperature, which leads to a substantial expansion and distortion of elementary lattices.

Currently available materials with good properties in acidic environments include steels with high contents of Ni, which makes these materials very expensive.

Another disadvantage of austenitic steels is that they usually exhibit a very low strength.

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