Materials of construction

Author: Katie Johnson ^[2015] , Helen Wu ^[2016]

Stewards: Jian Gong and Fengqi You

Material Properties

There are several properties of a material that can affect its suitability for the design. Before choosing a material, the designer should be aware of the following properties. Note that these properties for different common materials are often already collected and are available in various forms from manufacturers or in various textbooks.

Tensile Strength

The tensile strength, or tensile stress, of a material is the maximum amount of stress it can withstand before fracture. Proof stress, or yield stress, is similar, but measures that maximum amount of stress a material can withstand before deformation becomes permanent. Figure 1 below demonstrates where tensile strength (point u) and yield stress (point y) lay on the stress-strain curve for a material. There are standard tensile tests that measure tensile strength; however, strength is a common material property that is often already tabulated (Towler et al., 2013).

Figure 1: The stress strain curve for a typical ductile material (Engineering Archives)

In addition to considerations such as the pressure of the process, there are often guidelines that specify maximum allowable stress. One such set of guidelines is laid out by ASME in their Boiler and Pressure Vessel Code (Towler et al., 2013). This should be consulted while designing pressure vessels. There are also equations that can estimate these values. The maximum pressure that a cylindrical vessel can withstand is given by the following equations, where t is shell thickness, p is pressure, R is the inside vessel radius, and S is the allowable tensile stress:

$t={p*R \over {0.9*S-0.6*p}}$

$p={0.9*S*t \over {R+0.6*t}}$

There are tabulations of S for various metals found in Perry’s Handbook (Ulrich, 1984).

Modulus of Elasticity

The modulus of elasticity of a material, sometimes called its stiffness, measures the amount a material deforms when a certain amount of stress is placed on it. This measure applies when elastic deformation occurs, that is, when all deformation is reversible and is linearly proportional to stress (Callister et al., 2011). In Figure 1, the modulus of elasticity for the material would apply between the origin and point y. This is important because it measures the resistance of a material to bending and buckling (Towler et al., 2013).

Ductility

Ductility measures the amount a material will deform before it fractures (Towler et al., 2013). The equation for ductility is as follows:

$\%EL={{l_{f}-l_{0}} \over l_{o}}*100\%$ (Callister et al., 2011).

When a material has very low ductility it is defined as brittle. For example, in Figure 1 above, point f will be much closer to point u for a brittle material than for a ductile material. Brittle materials undergo very little deformation before they fracture, which means that in processes, there can be very little warming before a rupture. Some materials have a ductile-brittle transition points at low temperature. While these materials generally exhibit ductile properties, at low enough temperatures, they will not deform and will exhibit brittle fracture (Peters et al., 2003).

Hardness

The material’s ability to resist plastic deformation such as dents (Towler et al., 2013). There are many simple and relatively inexpensive tests, such as Rockwell Hardness Tests and Brinell Hardness Tests, which can determine the hardness of a material. It is useful to know the hardness of a material because it can be used to predict other mechanical properties such as tensile strength and can often be easier to determine (Callister et al., 2011). Figure 2 below shows an example of the correlation between Brinell hardness number and tensile strength.

Figure 2: Tensile strength versus hardness for various materials (VanAken, 2001)

Fatigue Resistance

Fatigue is failure of a material that can occur when there is cyclic loading on equipment, for example, in pumps. It can also occur if there are cycles of temperature or pressure (Towler et al., 2013). When there is cyclic loading, failure can occur at lower stress levels than the normal tensile strength. Fatigue failure is generally very similar to brittle failure with very little plastic deformation (Callister et al., 2011).

Other considerations

There are many other properties to consider while selecting a material. For example, creep is the amount a material deforms while it is under constant tensile stress over long periods of time and can especially be a problem for metals at high temperatures. Other considerations include the ease of fabrication, including welding ability and flexibility, the availability and cost of material, thermal conductivity (which is especially important for equipment like heat exchangers), electrical resistance, and magnetic properties for certain cases (Towler et al., 2013).

Process Considerations

Before choosing a material for a process, basic information must be collected, including temperature, pressure, and chemicals involved. The properties could affect the choice of materials.

Process Temperature and Pressure

In addition to knowledge of the average temperature a process with operate at, the engineer must be aware of the maximum and minimum temperature that could occur. While picking materials, the effect of temperature of material properties must be considered. Higher temperatures generally decrease the tensile strength and elastic modulus of metals (Towler et al., 2013).

Additionally, very low temperatures can cause some materials to brittle fracture. Therefore, if the minimum process temperature is below the minimum allowable temperature for a material, a different material (such as low temperature carbon steel) must be selected. Note that the expected, maximum, and minimum environmental conditions should be considered in addition to internal conditions.

Table 1: Recommended materials for strength at high and low temperatures (Biegler et al., 1997)

High temperature service		Low temperature service
T_max (F)	Steel	T_min (F)	Steel
950	Carbon steel	-50	Carbon steel
1300	330 stainless steel	-75	Nickel steel (A203)
1500	Stainless steels (304,321,347,316)	-320	Nickel steel (A325)
2000	Cast stainless, HC	-425	Stainless steels (302,304,310,347)

Similarly, the maximum and minimum pressures must be examined in relation to material properties. Different materials have different tensile stresses which affect that maximum pressure that can be used. When internal pressure is less than external pressure (that is, the process operates in a vacuum), either materials with higher allowable tensile stresses should be used or thickness should be increased.

Corrosion

If a process included certain corrosive chemicals such as oxygen, special allocations must be made for the materials. Additionally, environmental conditions such as salt from a nearby ocean should be considered. If corrosion is expected, the engineer should consider this in the material selection. This could involve picking a material that is naturally corrosive-resistant or by coating the inside of the pipe of equipment. These coatings can be made of paint or other organic coatings, especially for resistance to atmospheric corrosion (Towler et al., 2013). For internal protection, materials can be lined with rubber, glass, stainless steel or various polymers (Ulrich, 1984; Turton et al., 2012).

Types of Corrosion

Uniform corrosion is the most common type of corrosion and is considered the “general wastage of material,” while disregarding other sources of wear. Due to the nature of this type of corrosion, the corrosion rate can be predicted and experimentally determined. Corrosion testing is done to predict the penetration rate in inches per year, and can be expressed by the following equation,

$ipy=12w/tAp$

where w=mass loss in time t (lb), t=time (years), A=surface area (ft²), p=density of material (lb/ft³). Acceptable corrosion rates for carbon and low alloy steels are given in Table 2. For more expensive materials, these corrosion rates should be cut in half (Towler et al., 2013).

Table 2: Acceptable corrosion rates for low alloy steels (Towler et al., 2013)

	Corrosion Rate
	ipy	mm/y
Completely satisfactory	<0.01	0.25
Use with caution	<0.03	0.75
Use only for short exposures	<0.6	1.5
Completely unsatisfactory	>0.06	1.5

Galvanic corrosion is caused by the contact of dissimilar metals in the presence of an electrically conducting solution. This is caused by flow of current from the more active, anodic metal to the less active cathodic metal. See Figure 3 for the galvanic series of carious metals. This increases the corrosion rate, which is dependent on the relative exposed areas of the two metals. In order to minimize this corrosion rate, we should maximize the ratio of cathode to anode area. Another preventative measure is to electrically insulate the two metals in an attempt to break the conducting circuit (Perry et al., 2008).

Figure 3: Practical galvanic series of metals and alloys (Perry et al., 2008)

Pitting is a highly localized form of corrosion, resulting in the development of cavities or pits. This corrosion is much less predictable, and usually occurs in metals where the composition is not uniform. Pitting can be reduced with the use of a surface finish.

Erosion-corrosion can occur if the solution in contact with the metal contains suspended particles, or is of high velocity. In order to account for these factors and the resulting addition erosion, a more resistant material must be used at the surface.

Intergranular corrosion is a selective corrosion, occurring at grain or crystal boundaries of a metal or alloy. Even though the actual loss of material is small, it can cause failure of equipment by causing a loss of strength and ductility. This corrosion is very common with alloys, and rare with pure metals. It is caused by the presence of impurities existing at the grain boundary that accumulate during heat treatment.

Stress corrosion takes into account internal and external stressors that can cause premature failure of the material (Towler et al., 2013). Residual internal stressors include unequal cooling from high temperatures, structural rearrangements involving volume change, and stresses caused by rivets and bolts (Perry et al. 2008). External tensile stresses at the surface, along with the internal stresses, can cause this stress cracking. This is avoided during design by selecting materials that are not vulnerable in the specific corrosion environment and by eliminating high stresses (Towler et al., 2013). The source of stress corrosion is often due to the development of stresses during fabrication and welding, and can cause failure prematurely in as short amount of time as minutes or as long as years.

Liquid-metal corrosion occurs when liquid metals penetrate cracks between grain and weld boundaries. One example is mercury attack on aluminum alloys, and can cause catastrophic failure of the equipment (Perry et al., 2008).

Corrosion can also occur through high temperature oxidation for some low alloy steels. For these materials, the operation temperatures must be below 480 C to prevent oxidation and corrosion. There are ways to prevent this oxidation, such as creating a boundary between the solution and the material. Here, chromium is effective in resisting oxidation by creating an oxide film in between the solution and the equipment. Sulfidation is also a concern. Sulfur is a common element present in oil refining and energy plants. However, sulfur is very corrosive and can attack the equipment causing heavy corrosion.

Hydrogen embrittlement occurs at elevated temperatures and is the absorption and reaction of hydrogen in a metal resulting in the formation of methane. This results in the loss of ductility and cracking of the metal as decarburization occurs. Hydrogen attack is important to consider when specifying steels for use in hydrogen reforming plants. Alloy steels tend to have a greater resistance than plain carbon steels, but below 500 C, plain carbon steel can be used with limited hydrogen attack. Accepted limits for the use of carbon and low-alloy steels are shown in Nelson curves (Figure 4) (Towler et al., 2013).

Figure 4: Accepted limits for the use of carbon and low-alloy steels at different hydrogen partial pressures (Perry et al., 2008)

Cycling

Cycling occurs when a certain aspect of a process (temperature, pressure, or material levels) continually cycles between high and low levels. Cycling puts additional stress on the system and should be considered when selecting materials (Callister et al., 2011).

Common Materials

Metals

Carbon steel and stainless steels are some of the most common metals used in construction. Carbon steel is an alloy between carbon and iron. Also known as mild steel, carbon steel is one of the most commonly used engineering materials. It is favored because it is relatively cheap and widely available. It also has good tensile strength and ductility. However, carbon steel is not generally resistant to corrosion which can be an issue in many environments. When corrosion is expected, stainless steel is often favored. Stainless steel, especially with higher levels of chromium, is more resistant to corrosion (Towler et al., 2013). Stainless steel is also a better choice for low temperatures, as it has a minimum rating of -425 F, as opposed to the minimum rating of carbon steel of -50 F. Stainless steels are also a better choice than carbon steel when temperatures above 1000 F are expected (Biegler et al., 1997)

Other options include nickel and alloys, including Monel, a nickel-copper alloy (Ulrich, 1984). These are also corrosion-resistant to sulfuric and hydrochloric acids and salt water. Nickel-chromium alloys are good to chemical resistance at high temperatures (Turton et al., 2012). Copper and alloys have the advantage of corrosion resistance and good thermal conductivity. Thus, copper is often favored for heat transfer equipment (Ulrich, 1984). Aluminum and its alloys are more moderately priced than copper metals, are lightweight, and are better for low temperatures than carbon steel, however, they have lower strength and can also be susceptible to corrosion (Ulrich, 1984). Aluminum is often appropriate for cryogenic operations (Turton et al., 2012).

Table 2: Metal properties (Towler et al., 2013)

Material	Tensile strength (N/mm^2)	Modulus of elasticity (kN/mm^2)	Brinell Hardness	Specific Gravity	Max Allowable Stress (ksi)	Relative Cost
Mild (carbon) steel	430	210	100-200	7.9	12.9	1
Stainless steel	>540	210	160	8.0	20	2.0-3.0
Copper	200	110	30-100	8.9	6.7	22.8
Nickel	500	210	80-150	8.9	10	39.2
Monel	650	170	120-250	8.8	18.7	16.4
Titanium	500	110	150	4.5	10	6.8

Material Costs

Relative cost rating (Table 2) is determined by a set equation made with factors that normally determine a materials cost. This equation is

$CostRating={c*\rho \over {\sigma _{d}}}$

Where c is cost per unit mass ($/kg), ρ is density (kg/m3), and σd is maximum allowable stress (N/mm2).

This rating lets designers find what material will be the cheapest for the requirements needed for the design. More variables can be inserted in the equation depending on the design’s restraints (e.g. maximum temperature rating). As such, the rating is somewhat limited.

Materials with higher maximum allowable stress tend to be more expensive but can be used in smaller amounts.

Stainless Steels

Of all of the major metal types used in chemical design, the most commonly used and studied is stainless steel. Stainless steels are well known for the resistance to corrosion, making them highly applicable in the world of chemical processing wherein stainless steel pipes and tanks are used to contain corrosive liquids. Stainless steel is defined as a steel alloy with a minimum of 10.5% chromium by mass. Like steel, stainless steel comes in a variety of forms that can be made by altering its microstructure and then be further altered by changing the alloy's chemical makeup (i.e. adding or removing its elements). This flexibility of stainless steel allows it to be used in almost any application, depending on the requirements of the design. Generally, however, stainless steel is used when one needs the engineering properties of steel but with the added benefit of preventing corrosion (Biegler et al., 1997). Stainless steels work by containing the minimum necessary level of chromium to form a film of chromium oxide on top of the steel when in the presence of oxygen. This process blocks corrosion from spreading into the internal structure of the steel. Without this passive film creation, stainless steel would rust when exposed oxygen and water much like carbon steel does. However, there is a downside as the added benefit of stainless steel only works in the presence of oxygen. As such, stainless steels are not used in low air environments (SSIA).

Stainless steels come in three major types based on their microstructure: Ferritic, Martensitic, and Austenitic. Ferritic stainless steels have a body-centered cubic structure. They contain a wide range of chromium, anywhere from 13-20%, and no nickel with a maximum of 0.1% carbon. Ferritic stainless steels are usually less expensive and therefore are on the lower end of chromium content. This makes them less resistant to corrosion when compared to other stainless steels, but due to its structure it contains better engineering properties (Towler et al., 2013). Martensitic stainless steels are made from heating and quenching austenitic stainless steels to create a unique microstructure that is harder overall than austenitic stainless steels. This hardness can prevent weld decay, but it also makes the steel very brittle. Often, martensitic stainless steels are tempered to make a hard and tough material. The chromium content is around 10-12% with less than 2% nickel. Due to the low chromium and nickel content, martensitic stainless steels have relatively little corrosion resistance (SSIA).

Austenitic stainless steels make up more than 70% of the stainless steel produced around the world. They are distinguished by their face-centered cubic structure and carbides usage. The addition of nickel into the steel stabilizes the austenitic structure of the iron within. As such, austenitic stainless steels contain a minimum of 7% nickel with 18-20% chromium (Towler et al., 2013). They are popular due to their high corrosion resistance. Austenitic stainless steels cannot harden under heat treatment, which makes them susceptible to weld decay. Rather, they must go through the complicated process of martensitic treatment (SSIA).

There are “grades” of austenitic stainless steels that correspond to a specific chemical composition. The 200 series of austenitic stainless steels are typically more “general purpose” and cheaper as they substitute manganese for nickel. This makes them better rounded but less corrosion resistant. Type 201, for example, can harden under cold treatment (SSIA).

Type 300 series are more commonly used in chemical processes for their excellent corrosion resistance. Type 304, or 18/8 as its known for its 18% chromium and 8% nickel content, uses the minimum amount of chromium and nickel to still have its stable austenitic structure. This keeps its price low while still having excellent properties. The carbon content is usually low enough to prevent weld decay making the lack of potential heat treatment not a problem. Type 304L has even less carbon that type 304 (less than 0.03%) and is usually used in areas where carbide precipitation would occur for type 304 (e.g. thicker welded sections). For high temperature use, there is type 321, which is more temperature resistant and is a stabilized version of type 304. Types 309 and 310 can prevent oxidation at high temperatures due to increased chromium content. For even more corrosion resistance, type 316 is the best for its added molybdenum, which can thwart corrosion from dilute sulfuric acid (Towler et al., 2013).

Plastics

Plastics are becoming more commonly used when corrosion is expected. Plastics are also favored because they are inexpensive. However, they have low strength compared to metals (Ulrich, 1984). Plastics can be subdivided into several categories. The first of these is thermoplastic materials, which soften with increasing temperature. PVC falls within this category, and is the most commonly used thermoplastic material in chemical plants. The second category is thermosetting materials, which have a more rigid structure due to cross-linking. Rubber is also often used in linings for tanks and pipes (Towler et al., 2013). Table 3 lists some properties of common plastics.

Table 3: Mechanical Properties and Relative Costs of Polymers (Towler et al., 2013)

Material	Tensile Strength (N/mm^2)	Elastic Modulus (kN/mm^2)	Density (kg/m^3)	Relative Cost
PVC	55	3.5	1400	1.5
Polyethylene (low density)	12	0.2	900	1.0
Polypropylene	35	1.5	900	1.5
PTFE	21	1.0	2100	30.0

Inorganic Nonmetals

Inorganic nonmetals include glass, stoneware, brick and cements (Peters et al., 2003). Ceramics are generally stronger than other materials, especially at higher temperatures, they are much more brittle (Ulrich, 1984). Glass is good for corrosion resistance; stoneware is generally corrosive-resistant with more strength, but has poor thermal conductivity (Peters et al., 2003). Ceramic materials possess a cross-linked structure that may be crystalline or partly crystalline and possess a large range of properties across the different types.

Borosilicate glass, composed of silica and boron trioxide, is a very commonly used glass for chemical plants. Also widely known as “Pyrex,” this glass is stronger and can withstand high operating temperatures up to 700 C. In regards to pressure, borosilicate glass cannot withstand high pressures. Therefore, a safety precaution to consider during design is protecting the equipment with an external shielding, and allow for venting to the atmosphere to allow relief for any build up of pressure. Under these constraints, this material is often used for small-scale manufacture of specialty chemicals. It can be used to create pipes and fittings over a range of sizes, and is commonly used to manufacture distillation columns, absorption columns and heat exchangers. Glass is also often used as a lining, because of its relatively chemically inert properties. For example, glass is resistant to acids, salts and other organic materials. It is important to note, however, that glass is not resistant to alkalis and fluorine.

Stoneware is a better quality version of borosilicate glass, as it is stronger but carries similar properties and applications as borosilicate glass. It is often used for packing distillation and absorption columns.

Refractory materials are chemically and physically stable at high temperatures, therefore making them good contestants for manufacturing fired heaters and boilers. They are composed of silica (SiO₂) and alumina (Al₂O₃), and the varied balances of these two compounds strongly affect the performance of the material. Forming a eutectic system at 1545C of 99.45% SiO₂, it is important to keep the composition away from this breakdown. A eutectic system describes a homogenous solid mix, and at this point, the system will melt as a whole. Therefore, to ensure that the equipment will not experience distortion and melt, the composition of the refractories must be either above or below 99.45% SiO₂. Silica bricks contain >98% SiO₂, and are used for general furnace composition. Alumina bricks contain 60% Al₂O₃, and are used for special furnaces. Fire bricks, composed of 50% SiO₂, 40% Al₂O₃, balance CaO and Fe₂O₃ is also used for general furnace construction. One property of silica is that it can expand at certain temperatures, and therefore this needs to be taken to account during process design and operation (Towler et al., 2013).

Case Study

Chlorobenzene is produced by reacting liquid benzene with gaseous chlorine. The reaction takes place at 328K and 2.4 bar. If both the feeds are at 293K and atmospheric pressure, what are appropriate materials for the inlet piping and the reactor? (Adapted from Chemical Engineering Design (Towler et al., 2013)).

Both benzene and dry chlorine are not corrosive and therefore, carbon steel can be used as the inlet piping. Note that if the gaseous chlorine is actually wet chlorine, it becomes very corrosive to most metals and a plastic should likely be used. While the reactor is at higher pressure than atmospheric pressure, it is well below the maximum allowable stress for all common materials (Towler et al., 2013). However, a side product of the reactor is HCl which is corrosive. Likely, the HCl concentration will not be high enough to corrode the material but this should be investigated further. If concentration is >50% either in the reactor or later in the process, another material such as a plastic should be

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