There are Austenitic Grades (200 and 300 series), sometimes referred to as type 18-8. The 300 series, particularly types 304 and 316, are the most used grades in maintenance applications.

    • Grain Type of Austenite
    • Generally non-magnetic (will not attract a normal magnet)
    • Not hardenable by thermal treatment (Heat Treating)
    • Work-harden under impact, i.e. Cold work, strain hardening, etc
    • Generally functional up to about 1500F+ service temp.

There are Martensitic Grades (400 series most common)

    • Grain type of Martensite
    • Attract a magnet
    • Are hardenable by thermal treatment
    • Most often used where higher hardness and higher strength are required

There are Precipitation Hardening grades (PH Stainless)

    • Respond to a particular thermal treatment resulting in elevated strength properties
    • Considered to be an upgrade to the 400 series grades relative to toughness (combination of strength and ductility)

“Exotics” are the All Stars.

    • Special trade-name products.
    • Aquamet, Mir50, Waspaloy, Nitronic 50, Sea-Cure, Hastelloy, Custom 450, Greek Ascoloy.
    • Unique grades with elevated properties required for specific applications, and/or, conditions.

Their developers have sufficient confidence in their products to identify them back to the developer by means of a trade-name. Increased price may be of little or no concern relative to potential savings resulting from the advantages realized.

Duplex Stainless

    • Identified by a unique grain structure of austenite and ferrite
    • Most widely recognized modified group of stainless steel
    • Particular interest to industries that encounter sour service conditions
    • “Sour Service” is identified by the presence of Hydrogen Sulfide (H2S) in a content sufficient to cause cracking in steel parts.

A word of caution regarding improved grades of steel. THE DOUBLE-EDGED SWORD; As you improve toughness, hardness, corrosion resistance, etc., you create alloys that are better prepared to resist attack. They don’t differentiate between “adverse conditions” and intentional processing. User unfriendliness may be inherited as part of the bargain.You may get an advantage but one that requires significant precautions in machining, or welding, etc.

Some trade-name products have addressed user precautions and fabrication headaches and taken steps to minimize them. That will be listed as part of the advantage offered. Materials science and product development is happening daily. New products can be game-changers. Be as aware of the products user guidelines, as you are of the potential benefits. Make sure the steel will make the part, and the part will make it into service.

-Howard Thomas, February 1st, 2023

So, in our last post we briefly introduced Stainless Steel (SS) in general overview; We introduced the grades most common to general industry (304 and 316), and we looked at some of the properties of those grades.

There are, however, many grades of Stainless, each with its own strengths and weaknesses and economies. It is my feeling that due to overall education of the pros and cons of many of these grades, and lack of familiarity, many grades of stainless are used inappropriately in the maintenance arena. So many grades, so many mechanical forces, so many types of corrosion. (Variables relating to corrosive conditions alone are nearly infinite). One grade of stainless may be working in a particular application, but that may be only relative to other materials that have worked less effectively. So, let’s begin by getting acquainted with the benefits various grades may afford.

Two important things to keep in mind are; A specific grade, perfect for your application, may exist but may not be available in the form needed (tube, sheet, plate, bar). The grade may exist and be available, but, due to limitations in fabrication, may not readily lend itself to the configuration of your part.

Most often used, Type 304 and 316, fairly economical, good general corrosion resistance, elevated temperature resistant, and somewhat user ambivalent to a bit combative.
(Consider grade 303, if machining speed is a serious consideration, which is generally not the case in maintenance situations). These grades are austenitic 18-8 type, non-magnetic, will work-harden, but do not respond to thermal hardening.

Commonly used when increased strength is required; 410 and 416. Note that 416 is 410 modified for ease of machining. Superior to 304 and 316 in Sour Service Applications. (Martensitic, magnetic, responds to heat treating.)

Persistent problems, not addressed by either of the two groups listed above, will require consideration of modified, or enhanced grades. Those grades will involve modifications to element content (increasing content of elements, such as Nickel, Moly, Nitrogen, etc). Expect increases in cost. Among these are non-galling grades (such as Nitronic grades, Mir50), elevated temperature grades (such as 309 and 330), and a host of trade named products that are relatively economical and effective. Then there are, what I like to refer to as “The Exotics”; the next level up. Exotic in chemistry, and exotic in cost. Cost is of course relative to benefit; however, too often exotic materials are selected in an express leap to the ultimate solution. Generally, once that selection is made, the user is committed to a cycle of selecting grades equal or superior to that grade. Assumptions being, the grade in use is the minimum required grade. Going forward, service-life needs to be improved; Current service-life never being assumed to be the best-case scenario.

Modified grades may already be numerically identified (AISI SAE, UNS, etc.). Often, those targeting specific maintenance conditions, will be identified by a Trade-Name, identifying the producer. That producer will issue guidance on their trade named grades, to insure they are correctly applied in the field.

As you add to the chemistry and increase corrosion resistance, strength, and toughness, etc.,
you are creating a product that fights and resists forces that attack the part in service. Read that as a product that “Does not play well with others.” Keep in mind that when you need to work with it, it may not play well with you either. It may solve the problem in service, but you will have to get it into service first.

Pay close attention to “User Friendly” stainless. How does it machine? How weldable is it? Does it tend to gall? Does it move? Then take a second look at your part. User friendly stainless may be a bit of an oxymoron, but it can be as important as the ultimate in-service performance.

-Howard Thomas, November 18th, 2022