Piping Material and Metallurgy – Basic Guidelines for Selection: Part-1 of 3

  • Selection of basic Piping Metallurgy and Material (viz. CS, LTCS, AS, SS, etc.) for piping specification lies with Process/Metallurgy Engineer
  • The above selection is based on process, liscensor and/or intrinsic metallurgy requirement to suit process medium, like corrosion, high temp., pressure, etc.
  • This basic selection shall also address special considerations for PWHT, special valve trim for NACE, corrosive services like acids, amines, etc. and hazardous services like Hydrogen, Chlorine, Phosgene, Oxygen, etc.

This article will try to provide basic guidelines for piping material selection. The article is published in three parts. This is part 1 of the article.

Material Basics:

  • Metals rarely used in their purest form as they have low mechanical strength
  • Alloying helps increase its properties like strength and ductility. (Easiest eg. is adding Carbon to Iron to produce ferritic Carbon Steel)
  • Addition of alloying elements in proper proportions along with appropriate metal processing and heat treatment, results in optimization and improvement of it’s mechanical properties
  • Alloying also helps in improving corrosion & oxidation characteristics, machinability, weldability, etc.
  • Complex alloyed material is also being engineered for use in aero space programs and applications
  • Metallic glasses and crystalline alloys have also been developed and metal alloys are sometimes even bonded with graphite, ceramic and organic materials as composites for wider and complex applications

Mechanical Properties:

  • Modulus of Elasticity (Young’s Modulus) – ratio of stress to strain and measured using tension tests
  • Elastic range: Material returns to original shape after load is released
  • Plastic range: Material is permanently deformed even after load is released
  • Based on Stress-Strain Curve
  • Yield Strength – It defines the transition from elastic to plastic phase and it establishes the limiting value at which this transition occurs
  • Ultimate Tensile Strength – It defines the limit to which any further addition of load under constant strain would arrest the specimen elongation or thinning and would result in its failure.
  • Ductility – expressed in elongation of a specimen and its reduction in cross sectional area before it’s failure. Established by measuring specimen length before elongation and minimum diameter before failure.
  • Hardness – Ability of a material to resist deformation. Hardness is tested by Brinell or Rockwell Hardness tests, both of which are indentation type tests
  • Toughness – Ability of a material to resist sudden and brittle fracture due to rapid application of loads. Measured using Charpy V-Notch test.
  • Fatigue Resistance – Ability of a material to resist failure or crack initiation and its further propagation under repeated cyclic loading conditions

Terms and definition:

  • Creep Strength – Ability of a metal to withstand constant weight or force at elevated temperatures without yielding
  • Brittle fracture – sudden & rapid failure of a metal due to application of energy with hardly any deformation
  • Stabilization – Addition of alloying elements to prevent carbon-chromium precipitation and formation of carbides, which reduces corrosion at higher temps
  • Intergranular Corrosion (IGC)– Corrosion occurring at grain boundaries of metals due to depletion of chromium by formation of Cr Carbide layer, after reacting with carbon, which protects from further corrosive environments. (Min 12% Cr in SS). IGC is caused by reducing acids, oxidizing acids and organic acids
  • Reducing acids – In Chemistry reduction means loss of oxygen and gain of Hydrogen – examples are Hydrochloric acid, Hydrofluoric acid and hydrobromic acid
  • Oxidizing acids – Oxidation (chemical reaction between metal & Oxygen) means gain of Oxygen and loss of Hydrogen – examples are Sulfuric acid, Nitric acid and Chromic acid
  • Organic acids – are of carboxyl (COOH) group containing hydroxide (OH) – examples are Acetic acid, Formic acid, Citric acid
  • Stress Corrosion Cracking (SCC) – Failure of a metal through a combined action of tensile stress and chemical corrosion. SCC also depends on service temp, solution environment, exposure duration and metal properties
  • High Temp Hydrogen Attack – Results in degradation of Carbon and Low Alloy Steel due to depletion (decarburization) of carbon (strengthening agent) in steel due to reaction with Hydrogen at high temps, thus causing loss of strength in metal.
  • Hydrogen Blistering – A low temp phenomenon where atomic hydrogen diffuses into steel and is trapped as non-metallic inclusion, which builds up pressure and eventually bulges and blisters steel.
  • Hydrogen Induced Cracking (HIC) – Phenomenon similar to hydrogen blistering but HIC occurs in pipelines operating in sour services.
  • Hydrogen blistering and HIC can be controlled by restricting sulfur content in steel to 0.005% or 0.010% max.
  • Oxidation – Chemical reaction of metal and alloys with oxygen in the metal in air, to form oxides is called oxidation. This process results in scaling.

Effects of Alloying:

  • Carbon (C) – More carbon means more strength and hardness but less ductility and toughness
  • Phosphorus (P) – High content decreases shock resistance & ductility making material brittle
  • Silicon (Si) – Increases high temp properties making metal more stable by increasing tensile strength without increasing brittleness when under 2%. It also resists oxidization & is used as a deoxidizing agent
  • Manganese (Mn) – It improves hot working characteristics by increasing hardening when combined with sulfur
  • Nickel (Ni) – It improves hardenability by increasing the strength and toughness in steel. Combined with Chromium it improves impact and fatigue resistance. Improves low temp properties. Higher nickel content improves resistance to chloride cracking
  • Chromium(Cr) – It is a hardening element & improves material strength at higher temp. Improves high temp oxidation & corrosion resistance of steel
  • Molybdenum (Mo) – It makes the steel harder and stable by increasing its creep resistance at higher temp. 2% Mo in steel also reduces high temp oxidation rate
  • Columbium/Titanium (Cb/Ti) – Commonly used stabilizing elements to improve sustained high operating temp properties of steel by reducing carbide precipitation. SS Type 321 and 347

Typical Material Selection:

Cast Iron (CI) ASTM A126, A436:

  • Cast Iron/Ductile Iron/Malleable Iron – Are brittle, low strength material used for low temperature applications and basic utilities like air, water, drains, etc. Low cost material.
  • CI shall not be used on severe cyclic condition services, excessive heat, thermal shock
  • DI & MI cannot be used at temp below -29° C & above 343°C (ASTM A47, A536)
  • Austenitic DI (ASTM A 571) may be used at temp up to -196°C max but not lower

Carbon Steel (CS) ASTM A53-B/A106-B/API 5L-B:

  • Better than CI and has higher strength
  • Used for higher temperatures (up to 800° F or 427° C)
  • Most process services including steam piping

Low Temp Carbon Steel (LTCS) ASTM A333-Gr 1, 3, 4, 6, 8, 11, etc.:

  • Used for low temp services like chilled brine, chlorine liquid/gas, propylene, etc. (Bet -45°C to 485°C)
  • Has more of Carbon and no alloying elements like Cr and Mo and contains Nickel which improves low temp properties
  • Impact properties/values at low temp is better than in CS (Charpy N Notch test)
  • Refer to ASTM 01.01 for impact test requirement for low temp/cryogenic services

Galvanized Carbon Steel:

  • Use limited to about 200° F or 93° C for basic utilities like water, air, nitrogen
  • Normally piping connections are screwed to avoid damage to galvanizing due to welding

Lined Piping:

  • Normally metallic, glass, non-metallic, cement lined
  • Used for highly corrosive services like acids, caustic, process limited services, etc.
  • CS Cement lined pipe normally used in sea water applications

Alloy Steel (AS) – Also known as Cr-Mo steel:

  • Used for high temp applications in CS base like process services, superheated steam, reformer gases, etc. above 400° C design temp (ASTM A335 Gr P1, 5, 11, 22, etc.)
  • C-1/2Mo steel can be used bet -29°C up to 454°C design temp
  • Cr-1/2Mo steels can be used bet -29°C and up to 550 to 600°C
  • PWHT or stress relieving is a must after welding

Stainless Steel (SS) – Austenitic Grade Cr-Ni-Mo:

  • Used for high temp and process critical services and for cryogenic applications
  • Selection governed by process for specific service needs
  • ASTM A312 Gr TP 304 and 316 are normally used SS grades for pipes
  • Presence of 2% Mo in SS316 gives better overall corrosion resistance properties than SS304
  • SS316 has higher resistance to pitting and crevice corrosion in chloride environments
  • Grade L series has lower C (0.035%) which improves its use for higher temp up to 1100°F (600°C), has higher resistance to IGC and better weldability, Better mechanical strength at elevated temps           & good high temp oxidation resistance up to 925°C.
  • Grade H – Controlled C between .04 to 0.1% & lower Ni provides improved high temp strength above 815° C.
  • Common applications of SS304 are food, steel utensils, beverages, dairy industry, etc.
  • Common applications of SS316 are food, pharma, marine, medical implant steel, etc.
  • Grade 317 – use dictated by licensor/process
  • Grades 321 and 347 are metallurgically very stable in high temp applications because of the addition of Columbium and/or Tungsten
  • Impact testing is not required if C < 0.1%
  • Refer to ASTM 01.01 for impact test requirement for low temp/cryogenic services
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