© ATI Allegheny Ludlum Stainless Steels in the Process Industries John F. Grubb ATI Allegheny Ludlum October 2009

© ATI Allegheny Ludlum

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6 Stainless Steels in the Process Industries Overview Definition of stainless steels Review of SS families Overview of Alloying Elements Overview of the Nomenclature & Standards Basics of Corrosion Materials Selection

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Definitions Steel A material that conforms to a specification that requires, by mass percent, more iron than any other element Nickel Alloy A material that conforms to a specification that requires by mass percent more nickel than any other element.

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Definitions Stainless Steel A steel that conforms to a specification that requires, by mass percent, a minimum chromium content of 10.5 or more, and a maximum carbon content of less than (ASTM A 941)

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Definitions Passivity A condition in which a piece of metal, because of an impervious covering of oxide or other compound, has a potential much more positive than that of the metal in the active state. (ASM Metals Handbook)

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Definitions Passivity In stainless steels, it refers to a thin and protective oxide layer enriched in chromium, and where present in the base metal, also molybdenum and nickel.

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview Definition of stainless steels Review of SS families Overview of Alloying Elements Overview of the Nomenclature & Standards Basics of Corrosion Materials Selection

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Stainless Steel Families Austenitic - face centered cubic (fcc) series - Cr-Ni-Fe, Cr-Ni-Mo-Fe, etc series - Cr-Mn-Ni-Fe, etc. Ferritic - body centered cubic (bcc) - part of the 400 series - Cr-Fe Martensitic - grades in which martensite forms - other part of the 400 series - Cr-C-Fe

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Stainless Steel Families Duplex - ferritic-austenitic (two-phase) PH grades - grades in which a precipitation hardening reaction occurs

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Stainless Steel Families

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Stainless Steel Families Ferritic or Austenitic or Duplex? A Balancing Act Ferrite Formers Cr Mo Si Nb W Austenite Formers Ni N Mn C Cu

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview Definition of stainless steels Review of SS families Overview of Alloying Elements Overview of the Nomenclature & Standards Basics of Corrosion Materials Selection

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Alloying Elements Chromium (Ferrite Former) + primary element in the passive oxide layer for corrosion resistance, especially in oxidizing environments + increased oxidation resistance at high T - strong carbide former, also nitrides - key ingredient in intermetallic phases

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Alloying Elements Chromium (Ferrite Former)

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Alloying Elements Nickel (Austenite Former) + primary role is to promote the austenitic structure and the properties that austenite gives + increased high T strength + / - role in corrosion resistance

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Alloying Elements Molybdenum (Ferrite Former) + increases corrosion resistance in reducing environments, e.g. reducing acids + increases corrosion resistance in chloride environments, pitting, crevice, chloride SCC + increases high T strength - increases formation of intermetallic phases

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Alloying Elements Nitrogen (Austenite Former) + increases pitting / crevice corrosion resistance + increases strength from cryogenic to high T + reduces tendency to form intermetallic phases - formation of chromium nitrides in ferrite phase

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Alloying Elements Manganese (Austenite Former) + prevent hot shortness + increases solubility of nitrogen +/- complex behavior for Austenite formation and Martensite promotion/suppression - high temperature oxidation properties

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Alloying Elements Carbon (Austenite Former) + increases high temperature strength - formation of chromium carbides

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Alloying Elements Silicon (Ferrite Former) + increases fluidity of molten metal + deoxidizer + / - corrosion resistance Copper (Austenite Former) + corrosion resistance in sulfuric acid + precipitation hardening element

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Alloying Elements Sulfur + increases machinability + / - weldability - cracking due to hot shortness Titanium, Columbium (Niobium) + can tie up carbon - (Ti) Incompatible with nitrogen

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Alloying Elements Producing a new stainless alloy is not simple, as there are interactions with other elements and there is a need to be concerned with phase balance. In production, these may affect the melting and casting, hot working and cold working properties, and weldability. In end use, these may affect the stability over long term use, corrosion properties, etc.

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview Definition of stainless steels Review of SS families Overview of Alloying Elements Overview of the Nomenclature & Standards Basics of Corrosion Materials Selection

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Nomenclature & Standards Grade names - AISI grade names - e.g. 304L L = low carbon H = min. & max. carbon S = “Straight grade” - for many grades, 0.08% C max. N = Nitrogen F = high sulfur

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Nomenclature & Standards Grade names - AISI grade names SS committee no longer is active, therefore no new names for many years Names not unique among materials

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Nomenclature & Standards Grade names - UNS numbers - e.g. UNS S30403 Consist of one letter and 5 digits, unique to an alloy. Can be used internationally. Different metals have different letters. Stainless steels use the letter “S”, except castings which begin with J, a few high-nickel grades which begin with N and some weld filler metals which begin with W

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Nomenclature & Standards Grade names - UNS numbers - e.g. UNS S30403 Sometimes the numbers mean something, sometimes not – e.g., S32100 vs S32101 No mechanical properties (for stainless steels).

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Nomenclature & Standards Grade names - ASTM use grade types for stainless steel Use of common names and use of UNS numbers. Listing in ASTM A AISI names - common names - e.g., always a UNS number

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Nomenclature & Standards Grade names ASTM standards are product specifications ASME codes or standards give usage requirements - ASME uses ASTM-based standards and therefore grade names (types) - ASME also uses “Nominal Compositions” e.g., 16Cr-12Ni-2Mo for 316 and 316L - And UNS designations

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview of Nomenclature & Standards Grade names - Trade names - Names based on trade name - Grade names in other national standards Useful resources: Stahlschlüssel Woldman’s Engineering Alloys

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Trademarks are not specifications A trademark serves to identify the product of one supplier* and differentiate it from the product of others Material should be specified by trademark name only if the intent is to restrict supply to one producer __________ *A trademark can be used with the owner’s permission. If granted, this permission usually involves payment of a royalty or license fee.

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview Definition of stainless steels Review of SS families Overview of Alloying Elements Overview of the Nomenclature & Standards Basics of Corrosion Materials Selection

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Uniform or general corrosion Pitting Corrosion Crevice Corrosion Stress Corrosion Cracking Galvanic (dissimilar metal) Corrosion Intergranular MIC (Microbiologically Induced Corrosion) Erosion-Corrosion Corrosion-Fatigue

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Uniform or general corrosion Fairly uniform loss of thickness over the entire surface Fairly easy to measure corrosion rate and therefore to predict life expectancy and have corrosion allowance added to thickness Most stainless steels do not fail by general corrosion, but by localized corrosion

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Pitting Corrosion May be caused by chlorides, bromides or iodides Can initiate at surface defects, inclusions, etc. Once it starts, it most often continues growing because the environment at the bottom of the pit is different then the bulk environment

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Pitting Corrosion Pitting Resistance Equivalent PRE = %Cr X %Mo + 16 X %N Describes the relative resistance to the initiation of pitting, not the propagation It ignore factors related to inclusions (manganese sulphides) and surface condition, heat treatment, etc.

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Crevice Corrosion Can occur under all sorts of crevices metal-to-metal metal-to-gasket metal-to-plastic under deposits Driving force is difference in oxygen concentration

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Stress Corrosion Cracking There are many types of SCC possible for stainless steels - chloride (or other halogen) - caustic - hydrogen - polythionic

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Stress Corrosion Cracking Necessary conditions include: - tensile stresses - susceptible alloy - environment that can cause SCC (can be additional factors related to temperature, aeration, etc.)

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Galvanic (dissimilar metal) Corrosion Copper rivets on steel plate = No Galvanic corrosion Steel rivets on copper plate = Galvanic corrosion

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Galvanic (dissimilar metal) Corrosion - sounds quite simple but is actually quite complicated - most galvanic series are for seawater, but order may change with other chemicals - for the most part, no galvanic corrosion occurs between different stainless alloys, where both should be in the passive condition

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Galvanic (dissimilar metal) Corrosion - in general, use of stainless fasteners in non- stainless (carbon steel, copper, aluminum, etc.) structures is acceptable whereas use of non- stainless fasteners in stainless structures is risky

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Intergranular corrosion (IGC or IGA) Caused by formation of chromium-poor regions, typically by formation of chromium carbides Less of a problem now because of typically lower carbon contents, still an issue especially in strongly oxidizing acids L Weld

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion MIC (Microbiologically Induced Corrosion) MIC is a form of corrosion that occurs as a direct or indirect result of living organisms (microbes) in contact with a material The microbes do not “eat” the material, but they may need the material as a food. They may secrete fluids or create a condition where the material is attacked Almost all metals can have MIC corrosion

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Basics of Corrosion Types of Corrosion Erosion-Corrosion Corrosion-Fatigue There are many other types of localized corrosion that may occur on stainless steels, some together with the factors such as high velocity, presence of abrasives, or with mechanical stresses

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Overview Definition of stainless steels Review of SS families Overview of Alloying Elements Overview of the Nomenclature & Standards Basics of Corrosion Materials Selection

© ATI Allegheny Ludlum Stainless Steels in the Process Industries ALLOY SELECTION Several hundred stainless alloys exist About one hundred are produced Which one to chose?

© ATI Allegheny Ludlum Stainless Steels in the Process Industries PROPERTIES OF STAINLESS ALLOYS Stainless alloys have a multitude of outstanding properties, including: strength, toughness, ductility, and heat resistance Generally, the most important property is corrosion resistance

© ATI Allegheny Ludlum Stainless Steels in the Process Industries NECESSARY QUESTIONS What is Required? What is the Environment? What is the Cost of Failure? What is the Timing? (material availability) How sensitive is the product or process to Contamination? What about Upset Conditions?

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Selection of Materials Selection of materials of construction is a balance between the features and benefits of a material versus its capital cost. Corrosion resistance is not the only consideration that determines material selection. Cost considerations are always important, particularly in capital-intensive industries.

© ATI Allegheny Ludlum Stainless Steels in the Process Industries MATERIAL PERFORMANCE IS MULTI-DIMENSIONAL Strength Cost Corrosion Ductility Weldability Density Toughness Formability Stability

© ATI Allegheny Ludlum Stainless Steels in the Process Industries Availability is a material property In many situations, it is the most important property The alloy chosen must be available in a reasonable quantity, within a reasonable time, and in the product forms desired

© ATI Allegheny Ludlum Stainless Steels in the Process Industries ALLOY SELECTION Don’t over-specify: – Good enough is good enough – If it doesn’t corrode, selecting a material “twice as corrosion resistant” doesn’t provide any benefit But, do consider process upset conditions, accidents, and possible future operating parameters – Will increasing the temperature increase throughput? – What if cleaning solution is left in-place over a shutdown?

© ATI Allegheny Ludlum Stainless Steels in the Process Industries ALLOY SELECTION Don’t over-analyze: – Perfection is unattainable – That additional lab test may not provide a clear answer – There is a schedule to meet But do consider carefully the “necessary questions”

© ATI Allegheny Ludlum Stainless Steels in the Process Industries PITTING RESISTANCE EQUIVALENT Many PRE formulae have been developed based on data correlations PRE = Cr Mo PREW = Cr (Mo + ½W) PREN(16) = Cr Mo + 16 N PREN(30) = Cr Mo + 30 N MARC = Cr Mo + 20 (C+N) -0.5 Mn ­0.25 Ni

© ATI Allegheny Ludlum Stainless Steels in the Process Industries COMMENTS ON PRE PRE is a tool which may indicate potential corrosion resistance of an alloy, but is not a guarantee of corrosion resistance. Other factors (heat treatment, surface condition, etc.) must be considered as well

© ATI Allegheny Ludlum ECPT VS. ALLOY COMPOSITION G150 CPT VS. PRE AL 29-4C® AL-6XN® E-BRITE® 317LMN S21800 S32001 N=30

© ATI Allegheny Ludlum SCC IN BOILING 42% MAGNESIUM CHLORIDE – COPSON CURVE ™

© ATI Allegheny Ludlum SCC IN BOILING 42% MAGNESIUM CHLORIDE Boiling 42% magnesium chloride is not a typical environment Chloride content is very high pH is low Temperature is very high (above 150 °C) SCC failure in magnesium chloride does not prove susceptibility in other environments

© ATI Allegheny Ludlum SCC Resistance Oxygen- bearing, neutral chloride solutions N Cl¯, wt%

© ATI Allegheny Ludlum Physical Properties MaterialUNS Specific Heat J/kg  C Thermal conductivity W/m  C 20  C200  C400  C20  C200  C400  C FerriticS AusteniticS Duplex S S S Duplex SS density is about 7.8 g/cm3

© ATI Allegheny Ludlum Fig 4 Thermal Expansion per °C (20–100°C)

© ATI Allegheny Ludlum Mechanical Properties AISI/ UNS Yield Strength * MPa Tensile Strength* MPa Elongation Min,% Hardness* Vickers Type 304L Austenitic S Duplex S Duplex S Super Duplex * typical values

© ATI Allegheny Ludlum Summary Several hundred stainless alloys exist Stainless alloys have a multitude of outstanding properties, including: strength, toughness, ductility, and heat resistance corrosion resistance Materials selection begins with determining performance requirements.

© ATI Allegheny Ludlum Summary Selection of materials of construction is a balance between the features and benefits of a material versus its capital cost. Materials selection is rarely determined by one property. Don’t over-specify. Do consider process upset conditions, accidents, and possible future operating parameters.