WHAT IS CREEP?

Creep is a topic in metallurgy that can be quite confusing; while not common knowledge to many people, the subject of creep is one that those in charge of selecting metals must be aware of. Creep is a type of deformation, and metals that undergo excessive creep deformation can oftentimes fail completely. Unfortunately, creep-related failures happen frequently and are hazardous to both property and human life. Being informed during the metal selection process can help avoid metal failure due to creep and the serious consequences that come with it.

What is Creep?

Creep is a type of metal deformation that occurs at stresses below the yield strength of a metal, generally at elevated temperatures. One of the most important attributes of any metal is its yield strength because it defines the stress at which metal begins to plastically deform. Creep is unique in the fact that it is a phenomena that causes materials to plastically strain even though yield stresses have not been reached. The mechanics behind creep are complicated, but they can be broken down into three stages.

Stage 1: Primary Creep

Primary creep occurs first during the deformation process. At this stage, elastic deformation is initialized. Elastic deformation occurs from atomic bond stretching and is not permanent. Following the elastic deformation, permanent plastic deformation starts to take place. During the primary creep stage, this deformation occurs more rapidly at first and then slows with time. The reduction in the creep rate that occurs near the end of the primary creep stage is due to work hardening.

Stage 2: Secondary Creep

Secondary creep begins once the strain rate begins to stabilize and becomes constant. The strain during secondary creep occurs relatively slow when compared to the first stage and the third stage of creep. The creep rate remains constant and relatively slow because no microstructural damage has taken place yet.

Stage 3: Tertiary Creep

Tertiary creep is the final phase of the creep deformation process. This stage of the creep process begins once damage to the microstructure of the metal takes place. The strain rate accelerates as more and more deterioration of the microstructure continues to happen. After enough microstructural voids have been created, the metal eventually fractures and fails completely.

Common Instances of Creep

Creep is commonly found in some applications more than others. For instance, automobile frames are designed more with impact strength in mind since their static loads are small and normal operating temperatures are low. On the other hand, certain automobile engine components subjected to high loads and temperatures from engine combustion may experience creep if the right material is not selected.

Typically, applications that have high heat and high stress can be susceptible to creep. Examples include nuclear power generation, industrial engine components, heated metal filaments, jet engine components, and pressurized high-temperature piping.

How to Avoid Creep

The effects of creep can be circumvented or reduced through several different methods. One way to reduce creep is to lower the working temperature of the metal being used, although this is not always possible. Another method is reducing the constant load the metal has to withstand, but again, this may not be achievable depending on the application. Using a metal with large grains can reduce creep because less grain boundary sliding occurs. Certain metals with specific alloying element additions can avoid creep by eliminating microstructural vacancies.

WELDING PROCESSES FOR STAINLESS STEEL

Warped metal, charcoal-colored welds, and frustration; these are some of the things that can happen when welding stainless steel. While it contains similar elements to that of carbon steel, stainless steel has the addition of alloying elements such as chromium and molybdenum, and that presents an altogether different set of challenges when fusing two or more pieces of stainless steel rather than carbon steel. The oxygen surrounding the molten pool of stainless steel must be kept to an absolute minimum. The weld pool will behave differently than aluminum or carbon steel. The thermal conductivity of stainless steel is much less, making distortion and heat input a large concern. While there are many items to consider when welding stainless steel, one of the most important decisions to make is what welding process to use.

Welding Processes for Stainless Steel

Below we discuss the most popular welding processes used for stainless steel.

Metal Inert Gas (MIG) Welding/Gas Metal Arc Welding (GMAW)

MIG welding, or gas metal arc welding as it is more formally known, is one of the more popular ways to weld stainless steel. There are many similarities between MIG welding stainless steel and welding carbon steel. No special drive rolls need to be used, and the electrical polarity remains the same. However, shielding gas compositions are typically different. Lower amounts of oxygen are allowable when welding stainless steel, so O2 or CO2 levels should be kept around 2% or lower. It is quite common for tri-blend shielding gases that contain argon, helium, and carbon dioxide or oxygen to be used when MIG welding stainless steel. Since corrosion resistance will typically be desired in the weld as well as the base material, stainless steel welding wire must be used. Furthermore, to prevent cracking, the filler wire and base stainless steel should be a low carbon version or have stabilizers in them such as tantalum or niobium. Using a pulsed welding waveform can also help users MIG weld stainless steel more successfully.

Tungsten Inert Gas (TIG) Welding/Gas Tungsten Arc Welding (GTAW)

TIG welding, more formally known as gas tungsten arc welding, is another process that is frequently used to weld stainless steel. This process also has similarities between when it is used to weld carbon steel and when it is used to weld stainless steel. Both materials require a direct current electrode negative (DCEN) polarity. Typically, nearly 100% argon or helium shielding gases are used. As with MIG welding, TIG welding requires stainless steel filler metal to prevent making a weld that will be easily susceptible to corrosion. Low carbon or stabilized grades of stainless steel should be used as filler metals, and the base metals should also be low carbon or stabilized. Distortion can be a major problem when welding stainless steel, so it is important to keep travel speeds somewhat fast and heat inputs low when TIG welding stainless steel.

Flux-Cored Arc Welding

In general, welding processes that use flux are not optimal for welding stainless steel. That being said, it is possible to weld stainless steel with the flux-cored process. Special gas mixtures need to be used. Gas-shielded flux-cored arc welding is typically a better choice of process to weld stainless steel than flux-cored arc welding since it relies less on flux than the latter process to shield the weld metal from the atmosphere.

Metal-Cored Arc Welding

A better cored wire alternative to both self-shielded flux-cored arc welding and gas-shielded flux-cored arc welding is metal-cored arc welding. This is mostly because metal-cored arc welding does not rely on flux at all. The metal core of the filler material, while it does have certain kinds of deoxidizers, is mostly packed with powdered metals to increase deposition. With the proper shielding gas and wire feeding system, metal-cored arc welding can be used to make high-quality welds on stainless steel. For the most part, a pulsed waveform or spray-transfer arc is required to make a high-quality stainless steel weld with metal-cored arc welding.

Laser Beam Welding (LBW)

Laser beam welding is frequently used to join together stainless steel at very fast travel speeds and with very low heat inputs. Care must be taken to avoid porosity and cracking when welding with lasers. Cracks and porosity can be avoided through reducing the amount of oxygen via a shielding gas and weld parameter optimization. Laser beam welding is never performed manually, and therefore, must be automated if it is selected as the process to be used for welding stainless steel.

Other Welding Processes Used on Stainless Steel

The above-mentioned processes are perhaps the most common processes used to weld stainless steel. There are many other, somewhat less popular processes out in the industry that can be used to weld stainless steel. They include plasma arc welding (PAW), electron beam welding (EBW), shielded metal arc welding (SMAW), friction stir welding (FSW), and resistance welding (RW). This list is not exhaustive, and there are many more welding processes that can weld stainless steel together with varying levels of success.

WHAT IS A FERROUS METAL?

There are many different categories into which metals can be placed. Some metals may be ductile, some may be brittle. Other metals could be magnetic, and some have no magnetism at all. Some types of alloys can be precipitation hardened, and others cannot. The aforementioned are important distinctions; however, when categorizing metals, one of the most important differences worth noting is whether the metal in question is ferrous or non-ferrous.

What is a Ferrous Metal?

A metal with the descriptor “ferrous” means that it has iron in its composition. When the term ferrous metal is used, it also usually implies that iron is a large percentage of the elemental composition. If it’s not the most abundant element, it would probably be the second or third most prolific. If a metal only contains trace amounts of iron, as many metals do, then that small amount is not considered enough to declare the metal ferrous.

What are the Common Properties of Ferrous Metals?

It is difficult to assign common properties to ferrous metals, since they can have a wide variety of alloying elements that greatly change their characteristics. For instance, many ferrous metals are magnetic; however, it is not true for all ferrous metals. Austenitic stainless steel, while considered a ferrous metal, is not magnetic because the large amount of nickel allows it to have a crystal structure that is predominantly austenite at room temperature. Austenite is not magnetic, although it does contain iron. Some ferrous metals, such as cast iron, are extremely strong and brittle. However, low carbon steel, another type of ferrous metal, can be quite soft and ductile because it does not contain as high of an amount of carbon as cast iron.

While it is difficult to place the properties of all ferrous materials under one umbrella, there are some generalizations that can be made with some accuracy. Ferrous metals often have relatively high amounts of strength, especially when compared with copper, tin, and lead alloys. Ferrous metals are also generally hard, and if they’re not alloyed with many other elements or coated, can be subject to rust. Most ferrous materials, with the exception of austenitic stainless steel and some other grades, are magnetic.

Examples of Ferrous Metals

As was mentioned earlier in the article, there are many different types of ferrous metals.

The following are some examples of ferrous metals:

Carbon Steel

Stainless Steel

Cast Iron

Alloy Steel

Carbon Steel

Carbon steels are possibly the most widely used type of ferrous metal. They are primarily made up of iron, with over 90% of their chemical composition being that element. The only other major alloying element in carbon steel is carbon. There are only trace amounts of other elements. Common applications of carbon steels include structures, furniture, and automotive components.

Stainless Steel

Stainless steel is another group of ferrous metals that are commonly used. In general, stainless steels have a high amount of chromium that helps them to resist corrosion better than carbon steels. Stainless steels can be further broken down into subgroups. Austenitic stainless steels have the most corrosion resistance, with high amounts of nickel and chromium. There are also ferritic, martensitic, and duplex stainless steels. Each has their own advantages and disadvantages depending on the application. Common applications of stainless steels include appliances, pharmaceutical and medical equipment, food-grade equipment, and knives.

Cast Iron

Cast iron is a type of ferrous metal that has more carbon than most other types. This gives it a high amount of strength. Although high in strength, it is quite brittle. The lack of other alloying elements outside of iron and carbon make it a relatively affordable ferrous metal. Common applications of cast iron include cookware, small components subject to wear such as gears, rods, and pins, and mining equipment.

Alloy Steel

Alloy steels are a type of ferrous metal specially formulated to serve specific purposes. While composed primarily of iron, differing amounts of copper, vanadium, tungsten, manganese, and other elements can be used to tailor an alloy steel to have higher toughness, ductility, tensile strength, hardness, and other properties. Common applications of alloy steels include tools, dies, and machining equipment.

WHAT IS QUENCHING?

Heat treatment is a popular way to alter the mechanical properties of certain metals. Being able to change the hardness, toughness, and strength of a metal while keeping its chemical composition intact and virtually unaltered is a great way to tailor a metal to the needs of the environment and the demands of the job in which the metal is being used. There are many different ways to heat treat metal, one of the most popular ways is through a method known as quenching.

What is Quenching?

Quenching is a type of metal heat treatment process. Quenching involves the rapid cooling of a metal to adjust the mechanical properties of its original state. To perform the quenching process, a metal is heated to a temperature greater than that of normal conditions, typically somewhere above its recrystallization temperature but below its melting temperature. The metal may be held at this temperature for a set time in order for the heat to “soak” the material. Once the metal has been held at the desired temperature, it is quenched in a medium until it returns to room temperature. The metal also may be quenched for an extended period of time so that the coolness from the quenching process is distributed throughout the thickness of the material.

Quenching Media

There are a variety of quenching media available that can perform the quenching process. Each media has its own unique quenching properties. Considerations for the type of media use include quenching speed, quenching media environmental concerns, quenching media replacement, and quenching media cost. Here are the main types of quenching media:

Air

Oil

Water

Brine

Air

Air is a popular quenching media used to cool metals for quenching. Affordability is one of the main benefits of air; its affordability is a result of its profusion on earth. In fact, any material that is heated and then allowed to cool to room temperature simply by being left alone is considered to have been air quenched. Air quenching is also more intentionally performed when it is compressed and forced around the metal being quenched. This cools the part more rapidly than still air, although even compressed air may still cool many metals too slowly to alter the mechanical properties.

Oil

Oil is able to quench heated metals much more rapidly than compressed air. To quench with oil, a heated part is lowered into a tank that is filled with some type of oil. The oil can also be flushed through the part. Different types of oil are often used depending on the application because of their varying cooling rates and flash points.

Water

Water is able to quench heated metals rapidly as well. It can cool a metal even faster than oil. In a fashion similar to oil quenching, a tank is filled with water and the heated metal is submerged in it. It can also be flushed through a part. One benefit of water is that flammability of the media is not a concern.

Brine

Brine is a mixture of water and salt. Brine cools faster than air, water, and oil. The reason for this is that the salt and water mixture discourages the formation of air globules when it is placed in contact with a heated metal. This means that more of the surface area of the metal will be covered with the liquid, as opposed to air bubbles.

Quench Hardening Steel

Steel deserves a special mention when the quenching process is being discussed because its mechanical properties are very sensitive to quenching. Through a quenching process known as quench hardening, steel is raised to a temperature above its recrystallization temperature and rapidly cooled via the quenching process. The rapid quenching changes the crystal structure of the steel, compared with a slow cooling. Depending on the carbon content and alloying elements of the steel, it can get left with a harder, more brittle microstructure, such as martensite or bainite, when it undergoes the quench hardening process. These microstructures result in increased strength and hardness for the steel. However, they do leave the steel vulnerable to cracking and with a large reduction in ductility. For this reason, some steels are annealed or normalized following the quench hardening process.

PIPE SCHEDULE CHART

When working with pipe, it is common to describe them by their “schedule”. Someone unfamiliar with pipe schedules may not understand how to determine the measurements. A pipe schedule is a non-dimensional number that relates to the thickness of the wall and affects the inside diameter. It is used to help classify pipes for high or low pressure and temperature applications. To help, this blog features a pipe schedule chart, and explains how you can use it to determine the measurements and schedule of a pipe that you’re working with.

How To Use A Pipe Schedule Chart?

The conversion chart below is a great way to see the relationship between pipe size, schedules and wall thicknesses. If you’re looking to determine what schedule a pipe is, or what the nominal pipe size is, we suggest:

Measure the outside diameter and the wall thickness of the pipe

Refer to the pipe schedule chart and find the outside diameter

Find the wall thickness in the corresponding column

This will reveal the nominal pipe size and the schedule

For example, if you measure a pipe that has an outside diameter of 3.500 and a wall thickness of 0.300, using the pipe schedule chart you can determine that the pipe is a 3″ schedule 80 pipe.

Pipe Schedule Chart

Pipe Schedules & Weights
		Schedule 40		Schedule 80
Nominal Pipe Size	Outside Diameter	Wall Thickness	Weight Per Ft.	Wall Thickness	Weight Per Ft.
1/8	0.405	0.068	0.245	0.095	0.315
1/4	0.540	0.088	0.425	0.119	0.535
3/8	0.675	0.091	0.568	0.126	0.739
1/2	0.840	0.109	0.851	0.147	1.088
3/4	1.050	0.113	1.131	0.154	1.474
1	1.315	0.133	1.679	0.179	2.172
1-1/4	1.660	0.140	2.273	0.191	2.997
1-1/2	1.900	0.145	2.718	0.200	3.631
2	2.375	0.154	3.653	0.218	5.022
2-1/2	2.875	0.203	5.793	0.275	7.661
3	3.500	0.216	7.576	0.300	10.250
3-1/2	4.000	0.226	9.109	0.318	12.510
4	4.500	0.237	10.790	0.337	14.980
5	5.563	0.258	14.620	0.375	20.780
6	6.625	0.280	18.970	0.432	28.570
8	8.625	0.322	28.550	0.500	43.390
10	10.750	0.365	40.480	0.594	64.490
12	12.750	0.406	53.570	0.688	88.710

WHAT IS NORMALIZING?

It is important that the material used for any project possesses the correct mechanical properties for the specific application. Heat Treatment processes are often used to alter the mechanical properties of a metal, with one of the more common heat treatment processes being Normalizing.

What Is Normalizing?

Normalizing is a heat treatment process that is used to make a metal more ductile and tough after it has been subjected to thermal or mechanical hardening processes. Normalizing involves heating a material to an elevated temperature and then allowing it to cool back to room temperature by exposing it to room temperature air after it is heated. This heating and slow cooling alters the microstructure of the metal which in turn reduces its hardness and increases its ductility.

Why Is Normalizing Used?

Normalizing is often performed because another process has intentionally or unintentionally decreased ductility and increased hardness. Normalizing is used because it causes microstructures to reform into more ductile structures. This is important because it makes the metal more formable, more machinable, and reduces residual stresses in the material that could lead to unexpected failure.

What Is The Difference Between Annealing and Normalizing?

Normalizing is very similar to annealing as both involve heating a metal to or above its recrystallization temperature and allowing it to cool slowly in order to create a microstructure that is relatively ductile. The main difference between annealing and normalizing is that annealing allows the material to cool at a controlled rate in a furnace. Normalizing allows the material to cool by placing it in a room temperature environment and exposing it to the air in that environment.

This difference means normalizing has a faster cooler rate than annealing. The faster cooler rate can cause a material to have slightly less ductility and slightly higher hardness value than if the material had been annealed. Normalizing is also generally less expensive than annealing because it does not require additional furnace time during the cool down process.

The Normalizing Process

There are three main stages to a normalizing process.

Recovery stage

Recrystallization stage

Grain growth stage

Recovery Stage

During the recovery stage, a furnace or other type of heating device is used to raise the material to a temperature where its internal stresses are relieved.

Recrystallization Stage

During the recrystallization stage, the material is heated above its recrystallization temperature, but below its melting temperature. This causes new grains without preexisting stresses to form.

Grain Growth Stage

During the grain growth, the new grains fully develop. This growth is controlled by allowing the material to cool to room temperature via contact with air. The result of completing these three stages is a material with more ductility and reduced hardness. Subsequent operations that can further alter mechanical properties are sometimes carried out after the normalizing process.

What Metals Can Be Normalized?

To be normalized, a metal needs to be receptive to normalizing, meaning its microstructure can be altered by heat treatment. Many types of alloys can be normalized, including:

Iron based alloys (tool steel, carbon steel, stainless steel, and cast iron)

Nickel-based alloys

Copper

Brass

Aluminum

Common Applications for Normalizing

Normalizing is used in many different industries for many different materials. Examples include:

Ferritic stainless steel stampings in the automotive industry may be normalized following the work hardening that occurs during their forming process.

Nickel-based alloys in the nuclear industry may be normalized following the thermal microstructure alteration that occurs following welding.

Carbon steel may be normalized after it is cold-rolled to reduce the brittleness caused by work hardening.

WHAT IS AN ALLOY?

An alloy is a combination of a metal with at least one other metal or nonmetal. The combination must be part of a solid solution, a compound, or a mixture with another metal or nonmetal in order for it to be considered an alloy. The most common way to combine metals into an alloy is by melting them, mixing them together, and then allowing them to solidify and cool back to room temperature.

Why Are Alloys Used?

Metal alloys are used because they typically have enhanced mechanical or chemical properties. Alloying elements can be added to a metal to increase a number of properties including hardness, strength, corrosion resistance, machinability, and much more.

What Are Common Alloys?

Alloys are so abundant throughout the metalworking industry that there are too many to list. In fact, it is far less likely to work with a non-alloy, or “pure metal.” Even low carbon mild steel – perhaps the most frequently used material in metal fabrication – is an alloy of iron and carbon. An example of a steel alloy would be AISI 1018. Cast iron is another alloy of iron and carbon, with even higher amounts of carbon than mild steel.

Aluminum is often alloyed with other elements as well, giving it the attributes required for the desired application. For example, aluminum 6061 and 2024 have high additions of manganese or copper, respectively.

Alloys can also be extremely complex. Austenitic stainless steels, such as Grade 316, are a synthesis of iron, chromium, nickel, and some other metals and nonmetals. Bronze (which itself is an alloy of copper and tin) is often further alloyed with elements such as aluminum. Grade C954 is an example of an aluminum bronze alloy.

Tool steels like D2 are mostly made up of iron, but have many different additions of other metals and nonmetals such as chromium, vanadium, manganese, silicon, and carbon, depending on the desired mechanical properties.

What are Some Common Alloying Elements?

There are a wide variety of alloying elements that serve different purposes for different base materials.

Chromium is a metal frequently used to help alloys resist corrosion. Depending on the material, it can also increase hardness and strength.

Nickel is a metal often added to materials to increase toughness. Austenitic stainless steels have high additions of nickel which also acts as an austenite-promoter.

Copper is a metal used to make materials, such as aluminum, precipitation-hardenable. In steel, copper can increase corrosion-resistance, but can decrease the corrosion-resistance of aluminum.

Manganese is a metal usually alloyed to improve strength. Manganese alone as an alloying element is not affected very much by heat treatment, making it suitable for higher temperature applications.

Tungsten is a metal alloying element used to improve wear resistance (especially at high temperatures), toughness, and strength.

Lead is a metal alloying element that is used to improve machinability.

Silicon is a nonmetal alloying element. It is often used as a deoxidizer in metals. Silicon also increases strength and can reduce melting temperature.

Carbon is a nonmetal alloying element that is a necessary element to manufacture steel. Carbon additions are often used in steel and cast iron alloys to increase strength and hardness.

Want to learn more about common alloying elements? Check out our part 1 and part 2 of our “Common Alloying Elements” series.

WHAT IS TEMPERING?

Tempering is a heat treatment process that alters the mechanical properties (typically ductility and hardness) and relieves internal stresses of a steel. Tempering allows carbon trapped in a martensitic microstructure to disperse, and enables the internal stresses to be released from the steel that may have been created from prior operations.

The Tempering Process

Tempering is performed by elevating the steel to a set point below its lower critical temperature, typically following a hardening operation. Once this temperature is reached, it is held there for a specified amount of time. The exact temperature and time depend on several factors such as the type of steel and desired mechanical properties.

To get the steel to its critical temperature, some type of heating device must be used. Common devices include gas furnaces, electrical resistance furnaces, or induction furnaces. Often, this heating is done in a vacuum or with an inert gas to protect the steel from oxidation. Once the furnace achieves the desired temperature, a dwell time occurs. Following the dwell time, the furnace is shut off and the steel is allowed to cool at predetermined rate.

Why Is Steel Tempered?

Tempering steel after a hardening process allows for a middle ground of hardness and strength. This is achieved by allowing the carbon diffusion to occur within a steel microstructure. When steel is hardened, it can become excessively brittle and hard. However, when not hardened, the steel may not have the strength or abrasion resistance needed for its intended application. Tempering also improves the machinability and formability of a hardened steel, and can reduce the risk of the steel cracking or failing due to internal stresses.

When Is Tempering Used?

Tempering is most commonly used following a quenching operation. Heating a carbon steel and rapidly quenching it can leave it too hard and brittle. Tempering it can restore some of its ductility.

Tempering can reduce the hardness and relieve the stress of a welded component. Welds can create a localized zone that has been hardened due to the heat of the welding process. This can leave undesirable mechanical properties and residual stress that can promote hydrogen cracking. Tempering helps prevent this.

Work hardened materials often require tempering. Materials can become work hardened through processes such as punching, bending, forming, drilling, or rolling. Work hardened materials have a high amount of residual stresses that can be alleviated through a tempering process.

WHAT IS METAL FATIGUE?

Metal fatigue is one of the subtlest types of metal failures, and one of the most dangerous. Although metal failure can occur from excessive tensile loads, shear loads and impact loads, to name a few, metal fatigue is a failure type that can go unnoticed right up until the point when it is too late.

What is Metal Fatigue?

Metal fatigue occurs when metal parts are weakened due to repeated stresses. There are three stages to metal fatigue:

Stage One: After a certain amount of load cycles, micro-cracks begin to form on the metal during the metal fatigue process. The micro-cracks tend to form around stress concentrating geometric features such as edges. The stress required to create these micro-cracks can actually be less than the ultimate tensile strength and yield tensile strength of the metal.

Stage Two: These micro-cracks continued to be stressed by cyclic loading, causing them to increase in size.

Stage Three: Eventually, the enlarged micro-cracks reach a size where the stresses are enough to cause rapid crack propagation, leading to metal failure. The crack surface will be different in appearance depending on metal type and metal tensile strength.

Metal Fatigue Strength

In order to stop metal fatigue failures, it is important to understand how the characteristics of a metal affect its ability to resist fatigue. The most obvious and important of these is the fatigue strength of a metal. Fatigue strength is measured through fatigue testing, a group of evaluation methods that run material specimens of a determined size and shape through repeated cycles of a certain stress level. This is done for many stress levels, and a graph is plotted that highlights how many cycles, at a given stress level a material can withstand without fracturing. Being informed about the fatigue strength of a metal is extremely important to avoid metal fatigue failures.

Common Applications Subjected to Risk of Metal Fatigue Failure

Metal fatigue should be a consideration for all applications. However, there are several components and structures that are especially at risk for metal fatigue failure. Examples of some of these include:

Bridges

Automotive suspension equipment

Metal stamping equipment

Airplane body parts

High-vibration parts

Furniture

How to Prevent Metal Fatigue

Metal fatigue is a phenomena that can be lessened or altogether avoided with proper engineering considerations. One key method of preventing metal fatigue is by running a software fatigue analysis on component or structure designs. By running analysis and iterating the design process each time, metal failure can be avoided. For instance, software analysis could reveal an area subjected to substantial fatigue loads. If this was near a geometric feature such as a hole, the hole may be able to be moved elsewhere. Also, redesign could focus the stresses to an area that would be less subject to metal fatigue failure.

Another method to prevent metal fatigue is through appropriate material selection. Different materials have different fatigue strengths. For instance, steel can typically withstand higher amounts of stress cycles than aluminum can. By determining the fatigue strength of a material and evaluating how that will match up with the application, metal fatigue failure can be avoided.

WHAT IS ANNEALING?

While the chemical composition of a metal determines much of the mechanical properties, many metals can have their mechanical properties altered through heat treatment. There are many different types of heat treatment used today, and one of the most popular methods is annealing.

What Is Annealing?

Annealing is a heat treatment process used mostly to increase the ductility and reduce the hardness of a material. This change in hardness and ductility is a result of the reduction of dislocations in the crystal structure of the material being annealed. Annealing is often performed after a material has undergone a hardening or cold working process to prevent it from brittle failure or to make it more formable for subsequent operations.

Why Is Metal Annealed?

As mentioned above, annealing is used to reduce hardness and increase ductility. Changing these mechanical properties through annealing is significant for many reasons:

Annealing improves the formability of a material. Hard, brittle materials can be difficult to bend or press without creating a material fracture. Annealing helps eliminate this risk.

Annealing can also improve machinability. A material that is extremely brittle can cause excessive tool wear. Reducing the hardness of a material via annealing can reduce the wear on the tool being used.

Annealing removes residual stresses. Residual stresses can create cracks and other mechanical complications, and it is often best to eliminate them whenever possible.

What Metals Can Be Annealed?

To perform an annealing process, a material that can be altered by heat treatment must be used. Examples include many types of steel and cast iron. Some types of aluminum, copper, brass and other materials may also respond to an annealing process.

The Annealing Process

There are three main stages to an annealing process.

Recovery stage.

Recrystallization stage

Grain growth stage

Recovery Stage

During the recovery stage, a furnace or other type of heating device is used to raise the material to a temperature where its internal stresses are relieved.

Recrystallization Stage

During the recrystallization stage, the material is heated above its recrystallization temperature, but below its melting temperature. This causes new grains without preexisting stresses to form.

Grain Growth Stage

During the grain growth, the new grains fully develop. This growth is controlled by allowing the material to cool at a specified rate. The result of completing these three stages is a material with more ductility and reduced hardness. Subsequent operations that can further alter mechanical properties are sometimes carried out after the annealing process.

When Are Annealed Metals Used?

Common applications for annealed metals include:

Work-hardened materials such as sheet metal that has undergone a stamping process or cold drawn bar stock.

Metal wire that has been drawn from one size to a smaller size may also undergo an annealing process.

Machining operations that create high amounts of heat or material displacement may also warrant an annealing process afterward.

Welded components can create residual stresses in the area of the material exposed to elevated temperatures; to recreate uniform physical properties, annealing is often used.