BASIC GUIDE TO STAINLESS STEEL TYPES
STAINLESS STEEL TYPES Stainless Steels consist of five groups of alloys:•
AUSTENITIC
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FERRITIC
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MARTENSITIC
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DUPLEX
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PRECIPITATION HARDENING
FUNDAMENTALS OF SELECTION CORROSION RESISTANCE
Alloy selection will often be determined by the environment and the corrosion resistance expectation, or simply by comparison of alloys successfully used in similar applications or geographic location. Initial cost and service life may also be a factor. It is important to understand and appreciate, that many factors such as heat and chlorides can also impact service life. As a guide the basic way to determine corrosion resistance is to consider the alloy PREN number (Pitting Resistant Equivalent Number)
To improve the pitting corrosion resistance of stainless steels, alloying elements like molybdenum (Mo) and nitrogen (N) are added in certain grades. PREN is a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels based on the chemical compositions within an alloy Most commonly used formula:PREN = %Cr + 3.3(%Mo) + 16(%N) The table below indicates the typical range of PREN range for popular grades including higher alloy comparisons. Grade 304, 304L, 309, 310, 321 316, 316L 317, 317L S31803 - Duplex Alloy S31254 - 6 Moly Inconel速 alloy 625 Hastelloy速 alloy C-276
PREN 18.0 - 20.0 22.6 - 27.9 27.9 - 33.2 30.53 - 37.75 42.18 - 45.47 46.4 - 56.0 64.0 - 73.8
Inconel and Hastelloy trademark of Special Metals Corporation Table 1 As a general rule, the higher the PREN, the greater resistance the alloy has to corrosion. Notes:-
1. For Seawater Applications a PREN of 33 is normally considered the minimum. 2. Intamet stocks certain 316 Grades in higher Molybdenum variant 2.5 to 3% range. Words of note:PREN range given in table 1 is determined by the extremes of the alloy chemical content range within specification. Mid-range is a fair expectation; however, in practice it is normal to find stock material with the more expensive commodities such as Molybdenum, at lower end of range. Eg. 316L specification 2.00 – 2.50% Molybdenum, typical batch / heat content 2.05 – 2.15%. ARCHITECTURAL APPLICATIONS
“Tea Staining”, can occur as localised corrosion in certain environments where exposed to aggressive atmospheric conditions. Typically close to coastal areas, near airports and in heavy carbon dioxide cities. Fine tuning of specification is often overlooked. Initial cost versus product availability and planned maintenance, are all considerations. As a guide the higher the PREN, the higher the cost. To reduce the risk of such staining, enhancing the product finish to that of Mirror over Satin is worthy of consideration. Intamet offer a full polishing service.
STRESS CORROSION CRACKING (SCC)
SCC is quite a common problem in Austenitic grades “3 Series� above service temperature of 60 Deg C. with only traces of Chlorides. Ferritic grades are virtually free from any kind of SCC and Duplex grades are also highly resistant. Therefore, if SCC is important, both Ferritic and Duplex alloys are generally selected. MECHANICAL & PHYSICAL STRENGTH
Martensitic alloys and precipitation hardening grades are normally selected for high strength applications typically spindles and shafts, and predominantly supplied in bar form. In the UK the most specified grades being 431 and 17/4 PH respectively. These alloys hold inherent strength in excess of double that of 304 / 316 austenitic. FORMABILITY AND FABRICATION When selecting a material for formability, other factors often require consideration resulting in possible compromise. For example grade 303 is regarded as a free machining bar. With increased Sulfur offers excellent high speed machinability, however, the Sulfur compromises weld ability, formability and corrosion resistance against for example grade 304.
304 & 316 grades offer good formability and Weld ability, but do not machine as well as 303.
Reference to Mechanical and Physical properties is important selection criterion at the outset if formability is a part of the production process. COST
Intamet supply material from low cost commercial general purpose stainless steels, through to exotic super alloys. At an early stage of client specification, thought to life expectancy and total life cost should be key considerations. Consult with Intamet at an early stage to discuss availability of product and possibilities of tailoring chemical analysis to best meet the intended application. Intamet provide in excess of 15000 line items across a range of Stainless Steel and High Alloys.