Proper material selection plays an important role in refinery and petrochemical plants. All the possible outcomes shall be investigated thoroughly before taking any action. In oil and gas industry, pressure and temperature of equipment and lines my be too severe. In addition, most of the services are flammable and explosive. therefor, faulty decision can lead to a real disaster.  Stainless steels are designed for  higher durability in corrosive applications, so they can be found in any plant. As a result, there is a need for every petroleum engineer to get familiar with the introduction to stainless steel.

For every project, there is a document called Piping & Materials Specifications (PMS). In this document, all the available piping components with their acceptable materials are mentioned according to their services and operating conditions such as different pipes sizes, valves, fittings, etc. PMS is mostly developed based on ASTM alongside with ASME standard.

In this article we will focus our attention on stainless steels. The material which is intended to use in intense operating conditions!

Overlay, stainless steels are so famous for their excellent corrosion resistance. They are more expensive than regular carbon steel. They can withstand higher temperature. The erosion velocity limit is also higher. Carbon content of stainless steels is less than 1%. Chrome content is about 16- 26% while nickel content is 0- 22%. There are four major types of stainless steel which are different in compositions and atomic structures:

To review important steel’s terminologies, click here

Austentic stainless steel

Most common among others, non-magnetic, high toughness, high welding capability, high corrosion resistance. Examples are 304 (S 30400), 304L (S 30403), 316 (S 31600), 316L (S 31603). ASTM codes are A 312, A 240, A 409. The suffix “L” here indicates low carbon type (half of the regular type) which increases corrosion resistance on welding lines. The main difference between S 304 and S 316 is that S 316 contains molybdenum that increases both tensile strength & corrosion resistance.

Ferritic stainless steel

Magnetic, almost no nickel content, lower corrosion resistance, lower ductility, lower welding capability. Examples are S 409, S 420, S 430. ASTM code is A 240.

Martensitic stainless steel

Very high tensile strength, lower corrosion resistance. Examples are S 410, S 414, S 418. ASTM code is A 565.

Duplex stainless steel

Consists of both austentic & ferritic stainless steel, it is about twice as strong as others, very high corrosion resistance. Examples are S 318, S 327. ASTM codes are A 240, A 790.

Note that stainless steels (except duplex) are susceptible to chloride ions.

Effects of adding different elements in steel structure

In this section of “introduction to stainless steel” we specify how addition of single element can alter overall steel properties.

  • Carbon: Increases tensile strength. Decreases elongation and welding capability.
  • Nickel: Increases tensile strength
  • Chromium: Increases tensile strength and corrosion resistance
  • Molybdenum: Increases tensile strength and corrosion resistance. Decreases embitterment.