Pearliteproperties

The strain we apply up to the crossover (where the line changes from blue to green) is the yield stress.  As long as we don’t apply more stress than that, we’re causing elastic deformation and the part will spring back to its original shape when we remove that stress.  If we go past that point, plastic deformation occurs, and if we keep going, the part will eventually fracture or break.  Stress past that point is called plastic stress.

I will use the terms “strain hardening” and “work hardening” interchangeably throughout this article.  Let’s talk about this hardening more deeply.

The higher the n-value, the more easily the material work hardens.  17-4PH Stainless has an n-value of 0.05, which means it is only minimally work hardening.  But 304 Stainless has an n-factor of 0.45, which means it is very susceptible to work hardening.  I’m sure that’s one reason many machinists are not fond of it.

Some metals like aluminum and austenitic stainless steel cannot be heat treated or tempered to increase their hardness. But they can be work hardened by peening, rolling, forging, or drawing.

What is pearlitein steel

A value called the strain hardening exponent is one way to quantify the degree to which a material will work harden.  Here’s a table showing the strain hardening exponent (the n-value) for a number of common metals:

What is pearlitein iron-carbon diagram

Plastic deformation is the process of applying enough stress to the material so it is permanently deformed. The process of applying that stress is called “Cold Working” the material.

Bending a paper clip back and forth until it breaks is a demonstration.  The paper clip breaks because it eventually strain hardens so much from being bent, and is so hard that it becomes brittle and breaks.

Materials science has a lot to say about strain hardening, and it isn’t too hard to understand.  Let’s make it a touch more technical, so we have some language to discuss it.  Don’t worry, I have put all the really technical stuff at the very end of the article, so you can easily skip it!

Howis pearliteformed

Martensite is formed in steels when the cooling rate from austenite is sufficiently fast. It is a very hard constituent, due to the carbon which is trapped in solid solution. Unlike decomposition to ferrite and pearlite, the transformation to martensite does not involve atom diffusion, but rather occurs by a sudden diffusionless shear process. The term is not limited to steels, but can be applied to any constituent formed by a shear process which does not involve atom diffusion or composition change. The martensite transformation normally occurs in a temperature range that can be defined precisely for a given steel. The transformation begins at a martensite start temperature (Ms), and continues during further cooling until the martensite finish temperature (Mf) is reached. Ms can occur over a wide range, from 500°C to below room temperature, depending on the hardenability of the steel. The range Ms to Mf is typically of the order of 150°C. Many formulae have been proposed to predict the martensite start temperature. Most are based on the composition of the steel, and a selection are listed in the following table:

Pearlite is usually formed during the slow cooling of iron alloys, and can begin at a temperature of 1150°C to 723°C, depending on the composition of the alloy. It is usually a lamellar (alternate plate) combination of ferrite and cementite (Fe3C). It is formed by eutectoid decomposition of austenite upon cooling by diffusion of C atoms, when ferrite and cementite grow contiguously, C precipitating as Fe3C between laths of ferrite at the advancing interface, leaving parallel laths of Fe and Fe3C which is pearlite.

Beres and Beres [1] stated that their formulae were within 40°C of the actual Ms , in all cases studied, whereas other formulae had larger scatter bands. More recently, Ms models have been developed through the use of neural networks, trained on experimental data and using further data to validate and test the model, a reasonable approximation of Ms can be identified. Such models are available on the web [2] and can be used with compositional information. Neural networks based on the relationship between the chemical composition, transformation temperature and kinetics during continuous cooling enable calculation of a CCT diagram for the steel. These also take into account the influence of alloying elements on the phase transformation curves, as well as the resulting hardness. It is also possible to predict quantitatively the microstructure of the steel e.g. the percentage of ferrite, pearlite and bainite etc. [3]

Strain hardening, or work hardening as it is more often called, is the hardening of metal by plastic deformation.  It can be a problem or a benefit in metalworking.  Learn what it is and how to avoid or encourage it in this article.

What iscementite

Bainite is formed at cooling rates slower than that for martensite formation and faster than that for ferrite and pearlite formation. There are two forms of bainite, known as upper and lower bainite.

After strain hardening, the material will be somewhat less ductile, which may make it less suitable for its intended use. As the paper clip was bent it got harder, but less ductile, until it eventually broke.  The directional properties of the material may also be adversely affected.

The best tools at your disposal to avoid strain hardening are using proper feeds and speeds, tool coatings, and coolant. But we can add a number of other tips and techniques:

Strain hardening is caused by the dislocations in the crystal structure of the material running into one another. Annealing is the application of heat to force recrystallization that eliminates those dislocations.

Upper bainite generally forms at temperatures between 550 and 400°C. There are several proposed formation mechanisms, based on the carbon content and transformation temperature of the steel, resulting in slightly different morphologies. Low carbon steels exhibit fine bainitic laths, nucleated by a shear mechanism at the austenite grain boundaries. Carbon solubility in bainitic ferrite is much lower than in austenite, so carbon is rejected into the austenite surrounding the bainitic ferrite laths. When the carbon concentration in the austenite is high enough, cementite nucleates as discrete particles or discontinuous stringers at the ferrite/austenite interfaces. As the carbon content increases, the cementite filaments become more continuous, and at high carbon contents, the bainitic ferrite laths are finer with the cementite stringers more numerous and more continuous. The structure can appear more like pearlite, and is termed 'feathery' bainite.

Hardening a material sounds like a good thing, but there are times when it is undesirable. If nothing else, the material will be harder to machine, cut through, or form during the manufacturing process.  That means it may have to be cut with slower feeds and speeds, or that tool life will suffer.

Austenite was originally used to describe an iron-carbon alloy, in which the iron was in the face-centred-cubic (gamma-iron) form. It is now a term used for all iron alloys with a basis of gamma-iron. Austenite in iron-carbon alloys is generally only evident above 723°C, and below 1500°C, depending on carbon content. However, it can be retained to room temperature by alloy additions such as nickel or manganese. Similarly, ferrite was a term originally used for iron-carbon alloys, in which the iron was in the body-centred cubic (alpha- or delta-iron) morphology, but is now used for the constituent in iron alloys, which contains iron in the alpha- or delta-iron form. Alpha ferrite forms by the slow cooling of austenite, with the associated rejection of carbon by diffusion. This can begin within a temperature range of 900°C to 723°C, and alpha-ferrite is evident to room temperature. Delta ferrite is the high temperature form of iron, formed on cooling low carbon concentrations in iron-carbon alloys from the liquid state before transforming to austenite. In highly alloyed steels, delta ferrite can be retained to room temperature.

Think about this in relationship to machining operations.  A milling cutter slices and pulls out chips.  Sounds like the pulling out of the chip could be tensile deformation.  On the other hand, if we put the part in a hydraulic press we are applying compressive deformation.

The root cause of strain hardening is dislocation of the crystal structure of the metal.  This short video has great illustrations of what I mean by dislocations:

Pearlitemicrostructure

In theory, any metal or alloy can be strain hardened. What differs is how easily they strain harden and how much harder the get.

Pearliteformula

The application of heat must be controlled so the material does not melt. There are guides for most materials that tell you how hot it must get to anneal the material.

If we look at types of strain, we see two deformation mechanisms.  In tensile deformation, the length of the object increases.  In compressive deformation, the length decreases.

In-situ experimental studies based on synchrotron radiation can also result in valuable data to support computer models, as real-time study of such diffusionless phase transformations will be crucial to broaden the understanding of microstructural development and related structure-property relationships. [5]

Pearlitestructure

Heat is one of the biggest allies to work hardening. Increasing temperature increases the susceptibility of materials to work hardening.

Models combining the kinetics of martensitic transformation with mechanics, in view of microstructural development are also applicable. Finite element analysis enables evaluation of the local stress and strain fields as well as monitoring the kinetics of martensitic transformation and development of the understanding on critical parameters such as effect of austenite grain size on the resulting martensitic microstructure. [4]

Lower bainite generally forms at temperatures between 400 and 250°C, although the precise changeover temperature between upper and lower bainite depends on the carbon content of the steel. The transformation nucleates, like upper bainite, by partial shear. The lower temperature of this transformation does not allow the diffusion of carbon to occur so readily, so iron carbides are formed at approximately 50-60° to the longitudinal axis of the main lath, contiguously with the bainitic ferrite. With low levels of carbon, the carbide may precipitate as discrete particles, following the path of the ferrite/austenite interface. However, the overall mechanism of lower bainite formation is independent of carbon content in the main. The appearance of lower bainite strongly resembles that of martensite, but lower bainite is formed by a mixture of shear and diffusional processes rather than just shear.

Among the mechanical properties materials have, hardness is often a valuable one.  Various processes can be employed to increase metal hardness when desired, a process called cold working.

Elastic deformation is deformation that recovers when the stress is removed.  And yield stress is a mechanical property that is the amount of stress needed to hit the exact transition from elastic deformation (lower) to plastic deformation.  Yield stress is often the maximum allowable load on a component because applying more load will cause the component to be permanently deformed.

When iron carbon alloys transform from austenite on cooling, the solubility limit of carbon in ferrite is commonly exceeded. Under slow cooling conditions, carbides are formed, and at faster cooling rates carbon may be trapped in solid solution.

As machinists, we’re often concerned with how to minimize work hardening when working on a material prone to it.  More about that below.