Steel Archives - Industrial Metallurgists

Product design and manufacturing often involves trade-offs to optimize product performance, reliability, and cost. One aspect of cost includes the time and effort required to perform the manufacturing steps. In the case of fabricating carbon steel components, free-machining steels are often used to improve ease of machining. However, if components made of free-machining steel must be welded together, then issues arise, and costs rise, because of the poor weldability of these steels.

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The machinability of carbon steel containing up to about 0.5% carbon is improved by the addition of sulfur. Most resulfurized steels contain 0.08 to 0.13% sulfur, though some alloys allow sulfur content as high as 0.35%. The sulfur combines with manganese to form manganese sulfide (MnS) inclusions within the steel. These inclusions enhance the formation of microvoids during machining, which leads to the formation of broken chips rather than continuous chips. However, steels are typically specified to have a maximum of 0.050% sulfur because steel weldability decreases with increasing sulfur content. So, the number of MnS inclusions is small in standard grades of steel.

aMnS_inclusions

Manganese sulfide inclusions in steel

Phosphorous and lead may also be added to resulfurized low carbon steel. Phosphorus increases the strength and hardness of the ferrite phase in the steel. This promotes chip breaking rather than the formation of long, stringy chips. Lead is present as soft particles that enhance the formation of microvoids during machining.

Welding Free-Machining Steel
Free-machining steel with sulfur and phosphorous has poor weldability because the MnS inclusions and phosphorous compounds have a lower melting point than the steel. As the weld metal cools and solidifies stresses start to build across the weld due to shrinkage. Due to the presence of the still molten low melting point compounds within the weld metal the metal grains tear apart under the shrinkage stresses, resulting in solidification cracks.

The major concern with lead is its toxicity because lead can melt during welding and volatilize into the weld fumes. Occasionally, lead may cause weld porosity and embrittlement.

Welding free-machining steel is usually inadvisable because of these welding problems. If one of these steels must be welded, special electrodes must be used to try to reduce the sulfur and phosphorus content of the weld metal. Also, low welding currents should be used to minimize base plate dilution. Still, there is no guarantee that these precautions will eliminate the problem. Sometimes the best that can be achieved is a reduced level of cracking.

The amount of cracking that is tolerable will depend on the required service conditions and reliability of the weld. For a weld joint that will be part of a structural member, any amount of cracking is unacceptable. For a weld joint that is part of a structure that will bear light or no loads, some amount of cracking may be acceptable.

Design solutions to prevent weld cracking
Another solution to this problem is to select a standard carbon steel alloy with a composition, microstructure, and hardness that enables optimization between ease of machining and ease of welding, while meeting the product performance and reliability requirements.

Low and medium carbon steel for machining consists of ferrite and spheroidized cementite or ferrite and pearlite. Ferrite is a soft material and cementite is a hard material. Pearlite is a composite consisting of plates of ferrite and cementite. The amount of cementite or pearlite in a steel increases as the steel carbon content increases.


Ferrite and pearlite. The dark phase is cementite. The light phase is ferrite.

aFerriteAndSperoidizedCementiteSpheroidized and partially spheroidized cementite in a ferrite matrix.

Ferrite can be readily cut and causes little tool wear. However, it contributes to the formation of a built-up edge on the tool because of its low hardness. The presence of large quantities of massive cementite particles can cause significant wear on a tool since cementite is very hard. Pearlite is harder than ferrite and generally causes greater tool wear, with wear increasing as pearlite plate spacing decreases. However, a built-up edge is less common when machining pearlite than when machining ferrite.

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A few different heat treatments are used to obtain the different microstructures. Normalizing and full annealing result in ferrite and pearlite. Among normalized and annealed steels, those with lower hardness and smaller amounts of pearlite can be machined at higher speeds for equal tool lives. Spheroidization annealing is used to obtain ferrite and spheroidized cementite.

The machinability of as-rolled or annealed low-carbon steel improves with increasing pearlite content and with smaller ferrite grain size because microvoids form at the interface between pearlite and ferrite. Maximum machinability of low-carbon steels is achieved at 0.15 to 0.25% carbon in the as-rolled or annealed condition.

With medium carbon steels, normalizing or annealing, combined with cold drawing give a slight increase in machinability compared to as-rolled cold-drawn steel.

Cold working increases the hardness of ferrite, which results in shorter chip lengths and less built-up edge on the tool. However, using cold-worked steel may require a stress relief heat treatment prior to machining to minimize distortion during machining.

Imagine you’re a warrior during the middle ages and it’s time to get a new sword. So, you go to a blacksmith to buy a sharp, shiny long sword. A few weeks later you’re in a battle, fighting at the front of the shield wall. You take a huge swing at the enemy, who meets your blow with his sword, and your sword shatters into several pieces. Unfortunately for you, your blacksmith outsourced a batch of swords to a blacksmith on the other side of town who didn’t have time to temper the swords. As a result, the swords were strong, but brittle. Their lack of toughness meant that they could not absorb much of an impact before fracturing.

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Tempered martesite

Tempered martensite

Purpose of tempering

Tempering is used to improve toughness in steel that has been through hardened by heating it to form austenite and then quenching it to form martensite. During the tempering process the steel is heated to a temperature between 125 °C (255°F) and 700 °C (1,292 °F). At these temperatures the martensite decomposes to form iron carbide particles. The higher the temperature, the faster the decomposition for any given period of time. The micrograph shows a steel after substantial tempering. The black particles are iron carbide.

Untempered martensite

Untempered martensite is a strong, hard, brittle material. The stronger and harder it is, the more brittle it is. The strength and hardness is a due to elastic strain within the martensite, which is a result of too many carbon atoms being in the spaces between the iron atoms in the martensite. As the amount of carbon in a steel increases (up to about 0.8 weight percent carbon) the martensite strength and hardness increases.

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What happens during tempering

During the tempering process, the carbon atoms move out of the spaces between the iron atoms in the martensite to form the iron carbide particles. The strain within the martensite is relieved as the carbon atoms move out from between the iron atoms in the martensite. This results in an improvement in the steel toughness, at the expense of reduced strength.

Amount of tempering required

The amount of tempering required depends on the particular application in which the steel will be used. In some cases, toughness is not important, so tempering at a low temperature for a short period of time is acceptable. In cases where very strong and tough steel is required a high carbon steel tempered at a high temperature might be used.

More information about steel heat treating is in our online, on-demand courses Principles of Metallurgy or Metallurgy of Steel Heat Treating. The book Steels: Processing, Structure, and Performance by George Krauss provides a comprehensive discussion of steel heat treating.

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Abstract: This article discusses hydrogen embrittlement of carbon steel.  This includes a discussion of the mechanism by which a steel becomes embrittled by hydrgogen, circumstances that lead to embrittlement, the effects of embrittlement on steel behavior, how to prevent the embrittlement, and tests for evaluating whether a steel has been embrittled.

Hydrogen embrittlement is a metal’s loss of ductility and reduction of load bearing capability due to the absorption of hydrogen atoms or molecules by the metal.  The result of hydrogen embrittlement is that components crack and fracture at stresses less than the yield strength of the metal.

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Embrittlement process

At room temperature, hydrogen atoms can be absorbed by carbon steel alloys.  The absorbed hydrogen may be present either as atomic or molecular form.  Given enough time, the hydrogen diffuses to the metal grain boundaries and forms bubbles at the metal grain boundaries. These bubbles exert pressure on the metal grains. The pressure can increase to levels where the metal has reduced ductility and strength.

Hydrogen embrittlement

Situations leading to hydrogen absorption

Hydrogen can enter and diffuse through steel even at room temperature.  This can occur during various manufacturing and assembly operations or operational use - anywhere that the metal comes into contact with atomic or molecular hydrogen

Processes for which there is a possibility of absorption of hydrogen include acid pickling and electroplating.  Hydrogen is present in acid pickling baths.  During electroplating, hydrogen is produced at the surface of the metal being coated.  Acid pickling is used to remove oxide scale from the surface of steel and electroplating is commonly used to deposit zinc on steel nuts, bolts, screws and other fasteners for galvanic corrosion protection of the steel.  Other electroplated coatings are used for different applications.

Hydrogen absorption can also occur when a component is in service if the steel is exposed to acids or if corrosion of the steel occurs.

Intergranular Fracture

An example of failure due to hydrogen embrittlement is shown in the figures below.  The left image shows a macroscopic view of a fractured, zinc-plated, steel bolt.  The right image shows a scanning electron microscope image of the fracture surface.  In this image the individual grains at the metal fracture surface can be seen, which is indicative of intergranular fracture.  The bolt became embrittled during the zinc electroplating process.

Bolt and fracture surface

Intergranular cracking occurs when cracks form and grow along weakened grain boundaries in a metal.  In the case of hydrogen embrittlement, the hydrogen bubbles at the grain boundaries weaken the metal.

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Requirements for failure due to hydrogen embrittlement

There are three requirements for failure due to hydrogen embrittlement:

High-strength steels with tensile strength greater than about 145 ksi (1000 MPa) are the alloys most vulnerable to hydrogen embrittlement.

As mentioned earlier, exposure to hydrogen occurs during surface finishing process steps such as acid pickling and electroplating and during service if the steel is exposed to acids or if corrosion occurs.

As for the stress to cause fracture, even tensile residual stress within a component can be sufficient to cause failure of an embrittled material. 

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Preventing hydrogen embrittlement

Steps that can be taken to avoid hydrogen embrittlement include reducing hydrogen exposure and baking after electroplating or other processes that lead to hydrogen absorption.  Hydrogen embrittlement of electroplated components can be prevented by baking them at 375 to 430 °F (190 to 220°C) within a few hours after the electroplating process.  During baking, the hydrogen diffuses out of the metal.

For applications where there will be hydrogen absorption while a component is in service, the use of lower strength steels and reduction of residual and applied stress are ways to avoid fracture due to hydrogen embrittlement.

Evaluating for hydrogen embrittlement

Finally, there are tests that can be performed to evaluate whether processing leads to steel hydrogen embrittlement.  Here are two such tests:

The strength of metals is improved by impeding the motion of dislocations through the metals. One approach to achieving this improvement is to form a uniform distribution of closely spaced sub-micron sized particles throughout an alloy. The particles, which are called precipitates, impede dislocation motion through the alloy. Not every alloy can be precipitation strengthened. Alloys that can be precipitation strengthened include Al-Cu, Al-Mg-Si, Cu-Be, and 17-8 PH steel. The figure shows precipitates in a Al-Cu alloy.

Al2Cu precipitates in a Al-4% Cu alloy. © DoITPoMS Micrograph Library, Univ. of Cambridge

Al2Cu precipitates in a Al-4% Cu alloy. © DoITPoMS Micrograph Library, Univ. of Cambridge

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The particles are formed by precipitation, which involves a series of heat treatment steps. The first step is solution heat treatment. This involves heating the alloy up to a temperature that results in the atoms of the alloying element being dissolved within the solid structure formed by the array of atoms of the main element. For Al-Cu alloys, the copper atoms dissolve into the array of aluminum atoms. The dissolved structure is then retained at ambient temperatures by cooling the alloy rapidly, such as by water quenching.

After cooling, precipitates are formed either by natural aging or artificial aging.  With natural aging, the precipitates form at room temperature. With artificial aging, the precipitates form when an alloy is heated to a temperature lower than the solution heat treatment temperature. Only certain alloys will undergo natural aging. The other alloys must be artificially aged.

Regardless of the aging process, as the precipitation process proceeds the precipitates go through a series of stages, with changes in the size, form, and composition of the precipitates. The particular stage of the precipitates has a direct influence on the strength of the alloy. For artificially aged alloys, this is controlled by the aging temperature and time. At any particular aging temperature, there is an aging time at which the alloy will reach its maximum strength. This maximum strength corresponds to a specific stage of the form and composition of the precipitates. Aging for a time that is too short or too long will result in less than maximum alloy strength.

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For artificially aged alloys, the aging temperature affects the maximum strength that can be obtained, and the time required to reach maximum strength. For naturally aged alloys, the strength increases over time. The time required to reach maximum strength depends on the alloy.

Finally, precipitation strengthening can be combined with cold-working to give even greater alloy strength.

More information about the metallurgy of precipitation strengthening and precipitation strengthening heat treatment is in our Precipitation Strengthening course. Also, Heat Treatment: Structure and Properties of Nonferrous Alloys by C. R. Brooks, Precipitation Hardening by J.W. Martin, and ASM Handbook, Volume 4: Heat Treating discusses precipitation strengthening.

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The metallurgical phases present in an alloy have a huge impact on the properties of a metal component.  Phases are distinct materials that are comprised of the elements in the alloy.  These distinct materials have distinct properties that have an impact on the overall properties of the entire alloy.  Additionally, the size, shape, and location of the phases within the alloy also effect on the overall properties of an alloy.

Within many common alloys it is possible to alter the phases present with heat treatment.  Forming one or more phases from a different phase is called a phase transformation.   Phase transformations occur when in an alloy is heated or during cooling from an elevated temperature.

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There are several types of phase transformations that can occur when an alloy is cooled from an elevated temperature.  The type of transformation that can occur depends on the specific alloy.  Not all types of transformations occur in all alloys, and in some alloys no transformations are possible, other than the solid-liquid transformation.

Two of the most common phase transformations encountered with common alloys are eutectoid and precipitation.  For both types of transformation, the transformation involves the movement of atoms through the metal to rearrange themselves to form the new phase or phases.

Ferrite (white) and cementite (dark) in steel.

Ferrite (white) and cementite (dark) in steel.

A eutectoid transformation involves a change from a single phase to two other phases when the initial phase is cooled form an elevated temperature.  The most common alloy in which this transformation is encountered is steel.  The transformation occurs when steel is cooled from the austentizing temperature.  During slow to moderate cooling, the austenite transforms to ferrite and cementite.  The microstructure consists of cementite plates with ferrite between the plates.  This is commonly referred to as pearlite.  A micrograph of a steel alloy with 0.6% carbon is shown here.

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During faster cooling of some alloys, the ferrite bainiteforms in the shape of needles or plates and the cementite forms as particles.  This structure is referred to as bainite.  A micrograph of a steel with bainite is shown in Figure 2.

The reverse transformation occurs when steel with ferrite and cementite is heated.  When the temperature is high enough, the ferrite and cementite transform to austenite.  So, the austenite to ferrite + cementite phase transformation is reversible, and repeatable.

Precipitation transformations involve the formation of particles of one phase within an already existing phase.  These particles are called precipitates.  This transformation occurs when an alloy is cooled from an elevated temperature.  At the elevated temperature the phase present consists of the main element in the alloy with the alloying elements in solid solution.  When the alloy is cooled the solid solution is not able to hold all the atoms of the alloying elements in solution, so precipitates form that consist of the solute atoms and possibly the atoms of the main element in the alloy.

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Al2Cu precipitates in an aluminum matrix. © DoITPoMS Micrograph Library, Univ. of Cambridge

Al2Cu precipitates in an aluminum matrix. © DoITPoMS Micrograph Library, Univ. of Cambridge

For engineered metal components, precipitation  during cooling is undesirable because of the resulting size and location of the precipitates.  So, the process is modified by first quenching the alloy to room temperature to suppress the atom motion.  Then the alloy is either allowed to transformation at room temperature, if room temperature transformation is possible, or the alloy is reheated to an intermediate temperature to speed up the transformation.  An example of a common alloy system in which precipitation is used is the aluminum-copper system.  This figure shows a micrograph of Al2Cu precipitates in an aluminum matrix.

The precipitation transformation occurs in a number of alloys including aluminum alloys (Al-Cu, Al-Mg-Si, Al-Zn-Mg, and Al-Zn-Mg-Cu), precipitation hardened steels (e.g. 17-4 PH, 15-5 PH, and 13-8), some copper alloys (Cu-Be and Cu-Cr), and Zn-Al alloys.

Regardless of the particular transformation, control of the heating temperature, heating time, cooling rate, and, if necessary, reheating temperature and time are all important factors for controlling whether the desired transformation is complete and the shape, size, and location of the phases that form.  These in turn have a big impact on the properties of a metal component.  The relationship between heat treating process conditions, final microstructure, and properties is discussed in our Metallurgy of Steel, Metallurgy of Steel Heat Treating, and Precipitation Strengthening courses.

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