Metallurgy 101

Metallurgy 101Brought to you by

Houston Subsea Supplier Development

Joel Russo, CTG Metallurgist

Classes of Steels

• Carbon Steels – steels have no specified minimum quantity of alloying elements, basically just carbon and manganese. Very limited through hardenability. (e.g. 10XX XX = carbon equivalent)

• Alloy Steels – steels containing specific alloying elements other than carbon (typically chromium and moly)– A steel is considered to be an alloy steel when the

maximum of the range given for the content of alloying elements exceeds one or more of the following limits

• Manganese …………. 1.65%• Silicon ……………… 0.60%• Copper ……………… 0.60%

e.g. 41XX 41 = Chrome + moly, 43XX 43 = chrome + moly + NickelLow alloy steels are approved for H2S service at 22 HRC max. for H2S service, the steels must contain less than 1% nickel.

Free Machining Steels

• Free Machining Steels—contain elements that allow faster machining of parts. The main free machining additives are extra sulfur, phosphorus or lead. These materials have higher inclusion contents and lower toughness than regular steels. Free machining grades are not approved for H2S service per NACE MR0175/ISO 15156. Examples of these types of steels are 1144, 12L14, and 1215.

Classes of Steels• Stainless Steels – possess unusual ability to resist attack

by corrosive media at atmosphere and elevated temperatures. These properties are due principally to the addition of relatively large amounts of chromium, and also nickel and/or molybdenum. Need 11% minimum chrome to be a stainless.

• There are several main types of SS:– Austenitic—e.g. 18Cr-8Ni, 304, 316—nonmagnetic. These

stainlesses can not be hardened by heat treatment, but can be hardened by cold working. Approved for H2S service at 22 HRC max.

– Martensitic—e.g. 410, F6NM—magnetic. These steels can be hardened by quenching and tempering. Approved for H2S service at 22 HRC max for 410, and 23 HRC max for F6NM.

– Precipitation hardening (PH)—e.g. 17-4PH. Magnetic and heat treatable by solution annealing, quenching and aging. Can be hardened to over 180K yield

– Duplex SS – Dual phases-austenite (for corrosion resistance) and ferrite (for strength). Example 2205, F51-regular duplex yield strength approx. 60K. Super duplex – Zeron 100 (UNS S32760), 25-7 (UNS S32750). One problem with duplex is distortion during machining.

Nickel Based Alloys

• Nickel (Ni) gives phase stabilization, resistance to stress corrosion cracking (SSC) and general corrosion resistance. For example, 304 and 316 stainless steels are susceptible to SSC due to chlorides, at temperatures above 50 C.

• Chromium (Cr) gives resistance to oxidizing corrosives such as nitric acid.

• Molybdenum (Mo) gives resistance to pitting corrosion, due to chlorides, salts, and reducing acids, such as hydrochloric acid.

• Note: 945 is a new nickel base alloy produced by Special Metals Corp (SMC). It will be sold through Howco and Castle Metals. I have been told that stock will be arriving soon at Howco in the size range of 2” through 8” diameter. 945 is supposed to be priced slightly lower than 718, but this is dependent on quantities and market conditions. It is covered in FMC Materials Specification M40146.

Pitting Resistance Number

Nickel Based Alloys

Alloying Elements• Manganese (Mn)

– Generally present in all commercial steels– Essential for melting and rolling of steels– Combines with sulfur to improve hot working characteristics and to provide better

surface finish– Improves transformation of steel phases during heating and cooling– Major contributor to deep hardening

• Silicon (Si)– The major reason silicon is used in alloy steel is for its strong deoxidizer ability in

molten steel– Increases hardenability and strengthens low alloy steels

• Nickel (Ni)– Increases the internal strength and elastic limit of steels depending on the level of

carbon present in the steel– Steels containing nickel in sufficient quantities are very easily heat treated because

nickel lowers the critical cooling rate and allow quenched steel to attain higher levels of hardening

– Nickel containing steels have a greater resistance to impact at subzero temperatures– In combination with chromium, nickel produces alloy steels with higher elastic ratios,

greater hardenability, higher impact, and fatigue resistance than carbon steel

Alloying Elements

• Chromium (Cr)– Is a hardening element and is frequently used with toughening

elements such as nickel to produce superior mechanical properties

• Molybdenum ( Mo)– Steels containing molybdenum when hardened require higher

tempering temperatures to achieve the same degree of softness as compared to carbon or even other alloy steels. This ability to retain hardness is beneficial for applications where the material is subjected to relatively high temperatures

• Carbon (C)– When used with iron forms steel– Essential element for transformation– Typical surface hardness upon heat treating is increased with

increase in carbon content up to approximately 0.6% carbon.

AISI Numbering System

• A four numeral series is used to designate graduations of chemical composition of carbon steel, the last two numbers of which are intended to indicate the approximate middle of the carbon range.

Material Chemistries

Material Chemistries

Material Chemistries

Material Performance

Heat Treatment• An operation or combination of operations involving

the heat and cooling of steels in the solid state for the purpose of obtaining certain desirable mechanical, micro-structural, or corrosion resisting properties.

• Normalizing (1575ºF – 1725ºF) Typical 1650ºF– Provides grain refinement and uniformity– Performed after forging or hot working– The normalizing temperature should be the highest temperature in the

steel processing – Air cooled

• Annealing (1100ºF – 1450ºF)– Used to soften metals – Used to improve machinability– Cooled in furnace and is slower cooled than in normalizing

Heat Treatment• Austenitizing (1500ºF – 1650ºF ) Typical 1600ºF.

– Used to transform material to a harder phase– Usually followed by quenching

• Quenching– Rapid cooling to increase hardening– Quenching medium include water, oil, polymer. Use fastest quench that won’t

crack the material.– The higher the cooling rate of the quench medium the higher the hardness

• Tempering (Typically 950ºF – 1325ºF)– Tempering relieves stresses built up after quenching and insures better

dimensional stability– Material has temper, the softer the material lower yield.– The higher the temper, the softer the material, the lower the yield

strength, the lower the hardness, and the higher the toughness (e.g. charpy values)

– Lowering the tempering temperature increases the yield strength, increases the hardness, but lowers the toughness.

Heat Treatment

• Stress Relieving– Can be done after straightening operation to lessen stresses

induced after straightening bars.– Normally done at 50 to 100º F less than the tempering

temperature

• Quench Cracking Prevention– Avoid sharp corners, sharp radii and other stress

concentrations– Normalize after trepanning and prior to quench and tempering– Avoid quenching bar or metal parts with small diameter holes

(e.g. less than 2”)

Mechanical Properties

• Hardenability – The ability of a ferrous alloy to form martensite when quenched from a temperature above the upper Critical Temperature.– Hardness - Hardness is defined as the ability of a material to

resist permanent penetration. Depending on the method used, the larger/deeper the indentations made by a hardness tester the softer the material.

Mechanical Properties

– Brinell (HBW)• Approved by API (minor and major loads applied)• Most common method easily portable for field and large

parts• Indention diameter is measured and converted to Brinell

hardness units. – Rockwell HRC or HRB)

• Approved by API (minor and major loads applied)• Hardness is determined by depth of indention made by

constant load and not the diameter • Most accurate method, accepted over Brinell• B and C scales most common used

Material Hardenability

– Ranking of materials of lowest hardenability to highest. One method to assess hardenability is the Ideal Diameter (D.I.) Method. It is a way of quantifying the strength of the chemistry. The D.I. is the theoretical largest diameter bar that would through harden if quenched in an ideal quench. The major elements that affect the material hardenability are Cr, Mo, Ni, and V.

– 1018 (Range of D.I.:0.5” to 0.7”)– 1040 (Range of D.I.:0.8” to 1.2”)– 4130 (Range of D.I.: 2.5” to 3.5”)– 4140 (Range of D.I.: 4.0” to 6.0”)– 8630 Mod (Range of D.I.: 5.0” to 8.3”)– 4340 (Range of D.I.: 6.0” to 8.0”)– 4330V (Range of D.I.: 8.0” to 11.0”)– F22 (Range of D.I.: 9.0” to 11.0”)

Methods of Measuring Material Properties on Raw Material• Prolongation – An extension of the bar or forging, removed

after heat treatment. This gives a fairly accurate assessment of the strength and toughness of the material. Some codes require testing at mid radius, while others allow testing closer to the surface (e.g. 1.25” below the surface).

• QTC (Qualification Test coupon) – A QTC is approx. 4”X4” by 6” to 8” long. It does not give a true representation of the actual properties of the bar or forging unless the diameter is similar. Designers must take this into account. When the QTC accompanies the parts during the heat treat process, there can at least be some assurance that the parts were properly heat treated. Some codes allow the QTC to be heat treated by itself in a separate furnace. This is called a “capability” test and gives the least assurance of proper heat treatment of the parts.

Destructive Testing

– Tensile (Tension) Testing• Test specimen machined to specific size and shape (bar

bell) • Used to provide information of strength of materials• Results: yield strength, elongation, general strength,

reduction of area– Charpy Impact (V notch)

• Determines toughness at a specific temperatures • Test specimen machined to specific size and shape• .400” (10mm) x .400” (10mm) x 2” (standard) with “v”

notch in middle• Material tested at temperatures from ambient down to -

150ºF. Typical is -75ºF.

Tensile and Yield

Stress Strain Curve

Non-destructive Testing

– Magnetic particle testing• Ferrous material testing• Used to identify surface indications (cracks, etc.)

– Liquid penetrate testing• Ferrous or non-ferrous material testing• Used to identify surface indications

– Ultrasonic Testing• Ferrous and non-ferrous material testing• Used to identify indications below the material’s surface• Use of calibrated standards required

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