All matter, from a rock to an animal, to the magma at the center of the earth, is made from different combinations of substances known as elements.

The Periodic Table

The periodic table is a tabular display of all of the available chemical elements, organised on a basis of their properties. The elements more commonly used to make materials such as Cast Iron, SG Iron and Stainless Steel are: Iron (Fe), Chromium (Cr), Nickel (Ni), Carbon (C), Manganese (Mn), Magnesium (Mg), Silicon (Si), Phosphorus (P), Sulphur (S), Nitrogen (N), Molybdenum (Mo). This will be covered in more detail below.

Composition

The composition of a material (the elements used and their ratios), and the way they are processed determines the microstructure and consequently the properties of the material i.e. Strength, ductility (ability to deform under stress) and corrosion resistance.

Processing

It is important to understand how the material has been processed during the manufacture of a component as it affects the microstructure and properties of the material and hence how it will behave in your application. The correct processing is essential to achieve the required mechanical properties.

Most material processing operations involve a thermal history e.g. cooling from a high temperature shaping or deposition process, or producing a controlled diffusional change in a solid product.

A phase diagram provides some fundamental knowledge of what the structure of a material is as a function of temperature and composition. This is a complex topic in itself and is not covered here.

Further information on phase diagrams can be obtained at the following link from Cambridge University: Click Here

Typical Material Properties

Ultimate Strength: This is the strength of the material when it fractures.

Limit of Proportionality: This is the strength of the material when the relationship between stress and strain ceases to be linear. In low carbon steel this is classified as the “yield point”. Beyond this point the material does not return to its original length when the load is removed. Most designs do not stress materials beyond the limit of proportionality.

Elongation: How much the material has increased in length when it fractured. Good elongation properties, 15 to 20% are required for complex components which are highly stressed. Good elongation indicates ductility. Ductility is necessary so that components can deform very slightly to spread the load. A good cast iron may be 4%.

Reduction in area: Ductile materials “thin” slightly as they are stretched. When the material fractures, the cross sectional area of the fracture is less than the original. This is often reported in American standards but is rarely used in European standards.

Hardness: The ability of a material to withstand surface indentation. Several scales of hardness are used – Brinell Hardness Number, Vickers Pyramid Hardness, and Rockwell. In carbon steels, the hardness is directly related to the strength.

Impact Strength: The ability of a material to withstand shock or impact. Most materials lose impact strength as the temperature reduces. Two different test methods are used – Charpy (most popular), and Izod. A popular benchmark for offshore equipment is 27J at the design temperature. It is normal to check three test pieces.

Fatigue Strength: All of the tests defined so far are relating quick to carry out. Fatigue is different. A test piece is subjected to repeated tensile loads or repeated bending loads. Testing for fatigue is even more complex if it is required to be tested in contact with a specific liquid, vapour or gas. A typical test piece maybe classed as successful if it last for 5 million cycles hence a test may take a day or so to complete.

Creep Resistance: Creep is the permanent distortion of a material after being subject to a stress for a long period of time. It is not normally a problem in valves. It is more of an issue in hot components. Creep tests may run for years hence published research data is used when necessary.

Iron Based Materials

Types

Iron ore is a material that is dug out of the ground. When it is smelted with a high-carbon fuel such as coke, and usually a flux such as limestone, the resultant product is Pig Iron. Pig iron has very high carbon content (i.e. 3.5 to 4.5% wt %), which makes it brittle and of little use directly but is a starting point for making other iron types i.e.

• Cast Iron
• Ductile Iron
• Grey Iron (is grey in colour when fractured)
• Malleable Iron
• White Iron (is white in colour when fractured)
• Wrought Iron

Typical Valve Materials

“Alloying” a metal is achieved by combining it with one or more other metals or non-metals (elements from the periodic table) that often enhance its properties.

Cast Iron

Easy to cast and form. High strength but low ductility.

Iron (Fe) + Carbon (C) (2 to 4 wt%) + Silicon (Si)

SG Iron (Spheroidal Graphite Iron)

There are various types of Iron, one of which is ductile iron. SG Iron is a form of ductile iron, also known as Nodular cast iron or spheroidal graphite iron as graphite in ductile iron form in the shape of Nodules rather than Flakes as in Grey Iron.

Adding magnesium (Mg) to Cast Iron for example gives it ductility. SG Iron can be produced with a wide range of properties with increasing strength offset by decreasing ductility.

A typical chemical analysis for SG Iron would be:

Iron (Fe) + Carbon (C) (3.3 to 3.4%) + Silicon (Si) (2.2 to 2.8%) + Magnesium (Mg) (0.03 to 0.05%) + Manganese (Mn) (0.1 to 0.5%) + Phosphorus (P) (0.005 to 0.4%) + Sulphur (S) (0.005 to 0.02%)

Low Alloy Steels (e.g. Carbon Steel)

Cast Iron typically contains Carbon (C) at between 2 to 4% wt%.

Irons with less Carbon content are known as steel. Carbon Steel containing approximately 0.2% carbon and 1% manganese (Mn) can be used from -29degC to 425degC for many oil and gas applications.

Heat treating can be used to adjust the temperature window i.e. -46degC to 350degC.

For temperatures above and below these limits additional alloying elements are added. Nickel (Ni) improves the ductility for low temperatures, and Chromium (Cr) and Molybdenum (Mo) to provide resistance to creep and oxidisation at high temperatures.

Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. This may eventually cause the product to fail, in the case of a valve this could result in a rupture.

The correct heat treatment (processing) is an essential part of achieving the required mechanical properties of all low alloy steels.

Stainless Steel

Stainless Steel contains principally Iron and Chromium at a minimum level of 10.5%, which makes them more resistant to corrosive aqueous environments and to oxidation. At this level the chromium reacts with oxygen and moisture in the environment, to form a protective oxide film over the entire surface of the material. The oxide layer is very thin, typically 2 to 3 nanometres.

This layer exhibits a remarkable property in that when damaged it self-repairs as the Chromium in the steel reacts rapidly with the oxygen and moisture to reform the oxide layer. Increasing the level of Chromium beyond the minimum of 10.5% gives still greater corrosion resistance. Corrosion resistance maybe further improved as well as other properties, by the addition of 8% or more Nickel. The addition of Molybdenum further increases corrosion resistance, in particular resistance to pitting corrosion. The addition of Nitrogen increases mechanical strength and enhances resistance to pitting.

Although there are exceptions, stainless steel castings are classified as "corrosion resistant" when used in aqueous environments and vapours below 1200°F (650°C) and "heat resistant" when used above this temperature. The usual distinction between the heat and corrosion resistant casting grades is the carbon content. For a stainless steel casting to perform well in a corrosive environment, the carbon content must be low. Heat resistant grades have higher carbon contents to improve elevated temperature strength.

The chemical composition and microstructure differences between the wrought and cast versions of stainless steels can affect performance. Some stainless steel casting grades can be precipitation hardened by heat treatment, but the mechanical properties of most rely on their chemical composition.

The yield and tensile strengths of castings are comparable to their wrought equivalents. Cast stainless steels generally have equivalent corrosion resistance to their wrought equivalents, but they can become less corrosion resistant due to localized contamination, micro-segregation, or lack of homogeneity. For example, mould quality may cause superficial compositional changes that influence performance, and carbon pick-up from mould release agents can affect corrosion resistance.

Heat treatment and weld repair procedures can influence the performance of some cast grades and should be taken into consideration during grade selection.

Commonly Used Grades

Note: UNS material grade number, S signifies a wrought material grade, J signifies a casting grade. ACI stands for American Casting Institute.

For the casting grades, the letter “C” indicates the alloy is used primarily for corrosive service. If the first letter is a “H” the alloy is primarily for high temperature services at or above 1200degF or 649degC. The number indicates the level of carbon i.e. 8 indicates 0.08% Carbon and 3 indicates 0.03% Carbon.

Wrought producers make a significant quantity of 304 at less cost than 316, but for castings, foundries traditionally have standardized to only cast 316 (CF8M). The reason is that CF8M has much broader applications than CF8, so foundries produce a much greater volume of CF8M valves and benefit from the economies of scale.

Additional information on Casting Grades can be found here: Click Here

A much more in depth discussion about Stainless Steel can be found here (88 Pages): Click Here

Example Compositions

Stainless Steel

304 is the most common general purpose grade of stainless. 316 is the 2^nd most common grade and is widely used in the valve industry. The addition of the element Molybdenum (Mo) increases pitting corrosion resistance in the presence of chlorides e.g. salt. 304L and 316L is an extra low carbon grade, with an even higher resistance to specific forms of corrosion. 316Ti includes Titanium for extra heat resistance.

Note: Other grades are available.

Inconel

Inconel is typically used in high temperature applications. Originally developed in the UK in support of the development of the Jet engine. When heated, Inconel forms a thick stable oxide layer protecting the surface.

Note: Other grades are available.

Incoloy

Note: Other grades are available.

Monel

Monel was discovered in 1901 by the International Nickel Company and was named after the president of the company Ambrose Monell. Because of its high Nickel content it is classed as a Nickel Alloy. There are several commercial grades such as Alloy 400, Alloy 401, Alloy R-405, Alloy 450, Alloy K-500, Monel 404 Copper-Nickel Alloy.

Note: Other grades are available.

Hastelloy

Hastelloy is the registered trademark name of Haynes International, Inc. It is classed as a super alloy or high-performance alloy. The primary function of the Hastelloy alloys is that of effective survival under high-temperature, high-stress service in a moderately to severely corrosive, and/or erosion prone environment. Hastelloy alloys are used for many applications including pressure vessels, nuclear reactors, chemical reactors, as pipes and valves in chemical industry.

Note: Other grades are available.

Stellite

Stellite alloy is a range of cobalt-chromium alloys designed for wear resistance. Stellite alloy is very hard and is very difficult to machine hence its high cost. Often used on car engine valve seats and in gun barrels reducing the erosion from the hot gases. In the valve industry, should the application require, it is used to hard face valve trim’s and seats. Talonite is an alloy similar to Stellite.

Note: Other grades are available.

European Pressure Equipment Directive (PED 97/23/EC)

Because of the complexity of producing a material to the correct standard, PED 97/23/EC requires the material manufacturer to have an appropriate Quality Assurance System, certified by a recognised established body within the EU.

A material conformance certificate is a declaration from the manufacturer that the materials used comply with the specifications.

Valves compliant with PED regulations must be manufactured from traceable materials.

Certification is classified as follows:

2.1 - A Certificate of Conformance - The manufacturer certifies the materials conform to the order specification.

2.2 - A Works Report – The manufacturer certifies, on the basis of tests performed on the batch, the materials conform to the order specification. No actual certificates are supplied.

2.3 - A Works Certificate – The manufacturer certifies, on the basis of tests performed on the contract material, the materials conform to the order specification. No actual certificates are supplied.

3.1.A - Certificate issued by the manufacturers Quality Control or Inspection Departments – not someone from manufacturing stating the results of the actual material.

3.1.B - Certificate issued by an independent test house – employed by the manufacturer, stating the test results of the actual material.

3.1.C - Certificate issued by an independent test house – employed by the purchaser, stating the test results of the actual material.

A 3.1.B Certificate is the most commonly supplied certificate. Certification to 3.1.C is obviously the best from a purchaser’s viewpoint but is also the most costly and could have the biggest impact on delivery.