|7-Mo PLUS stainless is a duplex (two-phase) alloy with about 45% austenite distributed within a ferrite matrix. 7-Mo PLUS stainless possesses good resistance to chloride-stress-corrosion cracking, pitting corrosion, and general corrosion in many severe environments.The yield strength of annealed 7-Mo PLUS stainless is greater than twice that of typical austenitic stainless steels. 7-Mo PLUS stainless displays good corrosion resistance at weldments and superior impact strength when compared to 7-Mo® stainless.|
|This alloy should be considered for use in applications such as heat exchangers in petroleum refining, petrochemical chemical, pulp and paper, and allied processing industries as well as to replace 7-Mo stainless in existing facilities.|
|Elevated Temperature Use|
|7-Mo PLUS stainless is subject to 885 embrittlement when exposed for extended times between about 700 and 1000°F (371 and 538°C). Refer to the last two tables in the Mechanical Properties section.The alloy is also subject to precipitation of sigma phase when exposed between about 1250 and 1550°F (677 and 843°C) for extended time. Sigma phase increases strength and hardness, but decreases ductility and corrosion resistance.|
|General Corrosion Resistance:
The general corrosion resistance of 7-Mo PLUS stainless is superior to that of stainless steels such as Type 304 and Type 316 in many environments. Because of its high chromium content, it displays good corrosion resistance in strongly oxidizing media such as nitric acid. Molybdenum extends the corrosion resistance of 7-Mo PLUS stainless into the less oxidizing environments. Chromium and molybdenum impart a high level of resistance to pitting and crevice corrosion.Nitric Acid Resistance:
Reboiling of condensing nitric acid in the tubes of cooler-condensers can cause preferential attack occasionally in the weld and heat-affected zone of 7-Mo stainless. This type of corrosion has been simulated in the laboratory for comparison testing of welded tube sections (1.25″ [31.8 mm] OD x 0.065″ [1.65 mm] wall) of 7-Mo stainless and 7-Mo PLUS stainless. The 7-Mo PLUS stainless samples were significantly more resistant than the 7-Mo stainless samples when exposed to 50 w/o HNO3 in autoclaves at 300°F (149°C) for 800 hours. 7-Mo PLUS stainless showed essentially uniform general attack, whereas 7-Mo had severe intergranular attack in the heat-affected zone and light intergranular attack in the weld.Intergranular Corrosion Resistance:
Several laboratory tests have confirmed the superior resistance to intergranular corrosion of 7-Mo PLUS stainless. Samples of annealed strip and full-finished tube of 7-Mo PLUS stainless and 7-Mo stainless were tested in boiling 65% nitric acid (ASTM A262, Practice C). Although both alloys had corrosion rates of about 7 mils per year, the 7-Mo stainless tube samples suffered intergranular attack in the welds and heat-affected zones. The 7-Mo PLUS stainless tube samples had only uniform general attack. In a similar test, annealed Type 430 stainless had an average corrosion rate of 36 mils per year.
The outstanding corrosion resistance of as-welded 7-Mo PLUS stainless is shown in the following table. Strip and tube samples were tested in ferric sulfate-sulfuric acid according to ASTM A262, Practice B. Although this is a severe intergranular corrosion test for duplex stainless steels, the superiority of 7-Mo PLUS stainless is evident. Postweld heat treatment of 7-Mo PLUS stainless is unnecessary for most service environments. See hyperlink entitled “Typical Corrosion Resistance in Boiling ferric Sulfate-Sulfuric Acid” for details.
Because many stress corrosion failures initiate on the water or steam side of heat transfer equipment rather than on the process side, 7-Mo PLUS stainless should be considered a candidate for use when the corrosion resistance of Type 304 or 316 stainless steel is required for the process side and resistance to chloride- stress-corrosion cracking is needed on the water side. The following are applications where 7-Mo PLUS stainless may be a candidate for replacement of austenitic stainless steel tubing:
Chlorides on Process Side:
Chlorides on Water Side:
Limitation of Liability and Disclaimer of Warranty: In no event will South Coast Industrial Metals or any of its affiliates be liable for any damages arising from the use of the information included in this document or that it is suitable for the ‘applications’ noted. We believe the information and data provided to be accurate to the best of our knowledge but, all data is considered typical values only. It is intended for reference and general information and not recommended for specification, design or engineering purposes. South Coast Industrial Metals, Inc. assumes no implied or express warranty in regard to the creation or accuracy of the data provided in this document.