eBOOK WHAT IS STAINLESS STEEL? TABLE OF CONTENTS INTRODUCTION ...................................................................................................................................................................3 DISCOVERY OF STAINLESS STEEL .........................................................................................................................4 WHAT IS STAINLESS STEEL ..........................................................................................................................................5 CLASSIFICATIONS .........................................................................................................................................................6–9 Austenitic ......................................................................................................................................................................................6 Ferritic ............................................................................................................................................................................................6 Duplex ............................................................................................................................................................................................7 Martensitic ....................................................................................................................................................................................7 Precipitation Hardening............................................................................................................................................................8 Nickel Based Alloys ...................................................................................................................................................................8 Strength & Heat Treatment .................................................................................................................................................8–9 CORROSION ....................................................................................................................................................................10–18 What is Corrosion? ................................................................................................................................................................... 10 Basic Corrosion Resistance ................................................................................................................................................... 10 General or Uniform Corrosion ............................................................................................................................................... 11 Galvanic Corrosion ................................................................................................................................................................... 12 Pitting Corrosion ....................................................................................................................................................................... 13 Crevice Corrosion ..................................................................................................................................................................... 14 Intergranular Corrosion ....................................................................................................................................................15–16 Stress Corrosion Cracking ...............................................................................................................................................16–17 Microbiologically Influenced Corrosion ............................................................................................................................. 18 WELDING ................................................................................................................................................................................. 19 Welding Stainless Steel .......................................................................................................................................................... 19 Welding AL-6XN .................................................................................................................................................................19–20 Overalloying AL-6XN ........................................................................................................................................................20–21 Orbital Welding ......................................................................................................................................................................... 21 Autogenous Welding for AL-6XN ......................................................................................................................................... 21 ALLOY SELECTION .......................................................................................................................................................... 22 Conclusion ................................................................................................................................................................................. 22 REFERENCE CHARTS ..............................................................................................................................................23–25 2 WHAT IS STAINLESS STEEL? INTRODUCTION This is often the kind of statements heard from cause of the failure be identified before a proper individuals when discussing a failure of process fix can be recognized. Most often the cause of the piping or equipment. This is also an indication of failure is identified as the wrong material being used how little is actually understood about stainless steel in the wrong application. We can not solve problems and the applications where it is used. For years the by using the same kind of thinking we used when food, beverage and pharmaceutical industries have we created them. used stainless steels in their process piping systems. Most of the time stainless steel components provide This document provides engineers, purchasing satisfactory results. Occasionally a catastrophic agents, and plant personnel with a tool to enhance failure will occur. their knowledge of stainless steel and its uses as related to their present and future applications. The purpose of the information contained within this document is to bring an understanding to stainless steel, it’s uses, and why it will fail under certain conditions. In the following pages we will discuss the different classes of stainless steel, heat treatment, corrosion, welding, and finally material selection. As with any failure, it is imperative the THE DISCOVERY OF STAINLESS STEEL Stainless steel, the solid foundation of the sanitary table cutlery was either silver or nickel plated while industry, had a humble and practical beginning. In cutting knives were made from carbon steel that had 1912 while searching for a solution to erosion on to be washed and dried after each use. Then, the gun barrels caused by the action of heat and gases rust spots had to be rubbed off prior to the next use. during discharge, stainless steel was first discovered On his own initiative, Brearley, found local cutler R.F. at Brown Firth Laboratories. Mosley, introduced him to what he called “rustless steel” and had Mosley produce knives. When the However the first, true stainless steel was melted knives would not stain in vinegar, it was Mosley’s on August 13, 1913, and contained 0.24% carbon cutlery manager Ernest Stuart who called the new and 12.8% chromium. Building on the discovery knives “stainless steel.” from Brown Firth Laboratories, Harry Brearley was still trying to develop a more wear resistant steel. In The following year in Germany, Krupp was also order to examine the grain structure of this newly experimenting by adding nickel to a similar melt. developed steel, he needed to etch the samples The Krupp steel was more resistant to acids, softer before examining them under the microscope. The and more ductile making it easier to work. From etching re-agents Brearley used were based on these two inventions in England and Germany nitric acid, and he found this new steel strongly just prior to World War I, the 300 series austenitic resisted chemical attack. Encouraged, he then and 400 series martensitic steels were developed. experimented with vinegar and other food acids However, the discoveries and credits came full circle such as lemon juice and found the same results. in 1924 when Mr. Brearley’s successor at Brown Firth Laboratories, Dr. W.H. Hatfield invented 18/8 Brearley quickly realized this new mix would have a stainless steel. Containing 18% chromium and 8% greater product usage than just rifle barrels. He was nickel, it is commonly known today as 304 stainless from Sheffield, England, so he turned to a product steel. the region was known for: cutlery. Up until this point, 4 WHAT IS STAINLESS STEEL? WHAT IS STAINLESS STEEL? Stainless steel is not a single alloy, but a part of a table below, may be added for specialized purposes. large family of alloys with different properties for For example: high temperature oxidation resistance, each member. The stainless steel family is quite sulfuric acid resistance,greater ductility, high large and specialized. There are hundreds of grades temperature creep resistance, abrasion resistance, and sub grades, and each is designed for a special