Why Cold Steel Is Brittle

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Effect of Temperature on Strength

Steels become stronger at lower temperatures. This can be measured with the “yield strength” which is the load to permanently deform the steel. This deformation is in the form of a shape change, ie. if you are bending steel it stays bent, when hammering hot steel it dents, when flexing an edge it rolls. This is perhaps easier to visualize for those that have forged steel because hot steel is easier to forge, and is progressively more difficult to forge as it cools down. This increase in strength at lower temperatures continues below room temperature, so steel at cryogenic temperatures is stronger than at room temperature. Here are values for yield stress for 410 stainless steel heat treated to 39 Rc [1]:

Toughness

Toughness is a measure of resistance to cracking and fracture. One common method for measuring toughness is with the impact test where a heavy hammer on a pendulum is dropped through a specimen which breaks it. If the sample absorbed no energy the hammer would reach the same height as it started on the other side, which would indicate zero toughness. Different heights indicate different levels of toughness from the different amounts of energy the specimen absorbed.

Low Temperature Toughness

In general, higher strength and higher hardness means lower toughness. Therefore we would expect steel at low temperatures to have lower toughness than room temperature steel. This may be concerning if knives are to be used outdoors in the winter, particularly in northern climates. However, this is not always a gradual reduction in toughness with lower temperature, with martensitic steels like knife steel there is often a sharp reduction in toughness below a certain temperature. Here is a plot showing the change in toughness with temperature for steels with different carbon content [2]:

The temperature at which this drop in toughness occurs is called the “Ductile to Brittle Transition Temperature” (DBTT) which is about -75°C for the 0.01% carbon steel above. Each steel has an “upper shelf” toughness at higher temperatures and a “lower shelf” toughness at low temperatures. The upper shelf toughness goes down with more carbon. The higher carbon steel is harder/stronger and therefore has lower toughness. The drop in toughness with temperature is more gradual with higher carbon. Because the drop in toughness can be gradual it can be difficult to identify a single transition temperature. One convenient definition is the temperature at which the toughness is an average of the upper and lower shelf.

You can also see that for high carbon steels the DBTT is actually above room temperature. This is somewhat misleading as the toughness test is performed with notched charpy specimens; the notch promotes brittle fracture of steel and reduces toughness:

Austenite vs Martensite

Not all materials show the DBTT with the sharp drop in toughness. Steels with a ferrite or martensite microstructure show this behavior. Ferrite is the steel phase in the annealed condition and martensite is the steel phase in the quenched and tempered (hardened) condition. Steels with an austenite microstructure (ie austenitic stainless steel like 302, 304, etc.) do not show this behavior. See the chart below and compare the FCC material (austenite) with the BCC material (ferrite and martensite):

With high strength materials like knife steels the gradual drop in toughness is in part because the relative toughness is so low already. I don’t know if the decrease in toughness is more sharp with unnotched specimens that start out with higher toughness. A literature search has not revealed anything either. However, we can talk about the mechanisms being measured by this toughness testing and what we might conclude. 

Why the Ductile to Brittle Transition Occurs

As described earlier, the yield strength (σ_ys) of steel increases as the steel becomes colder. Another parameter of importance is the fracture strength (σ_f), or the stress required to grow a crack. Fracture strength is less affected by temperature than yield strength is. Therefore, with a reduction in temperature the yield strength increases while the fracture strength is largely the same. The temperature at which the yield strength is higher than the fracture strength is the point at which fracture is “easier” than deformation. This point is the Ductile to Brittle Transition Temperature:

This transition from deformation to pure fracture is seen when analyzing the fracture surfaces of charpy specimens with different temperatures. When the yield strength is lower than the fracture strength, the sample deforms as it is broken. In other words, it fractures in a ductile manner. When the yield strength is higher than the fracture strength, it fractures in a brittle manner without any deformation. With toughness testing across a range of temperatures, the amount of deformation can be seen visually with the broken specimens, with the 10% specimen being brittle and the 100% specimen being ductile. One definition of the Ductile-to-Brittle Transition Temperature is 50-50 ductile and brittle fraction. 

Factors that Change DBTT

There are many factors that can change the DBT temperature. We want the temperature to be as low as possible to both maximize room temperature toughness but also to increase cold temperature toughness. Carbon is a major factor as described, but there are many other variables to keep in mind.

Impurities

Common impurities sulfur and phosphorous reduce the fracture strength of steel and reduce its toughness. This also increases the DBTT:

Grain Size and DBTT

A smaller grain size in steel increases its toughness. I previously wrote about this in How Does Grain Refinement Lead to Improved Properties? The many small grains means that the fracture strength is increased. When a crack is growing through steel, each time it meets a grain boundary the crack has to re-initiate, and therefore many small grains means fracture is more difficult. 

A finer grain size also means a higher yield strength, for reasons described in the linked article. Therefore, with both an increase in yield strength and fracture strength, the DBTT is reduced:

Nickel and Manganese Additions

Some alloy additions can make the steel’s yield strength less sensitive to reduction in temperature. Therefore the temperature where the yield strength crosses over with the fracture strength is reduced. One common one is manganese:

One alloy addition perhaps more familiar to knife steel nerds is nickel, which can improve room temperature toughness but is best known among metallurgists for improving low temperature toughness:

While the studies shown above are for relatively low carbon steels, nickel also improves toughness in knife steels, such as with L6 tool steel which has a 1.5-2% nickel addition:

Perhaps it is the decrease in DBTT closer to room temperature that is what provides the improvement in toughness from nickel in knife steels like L6, since high carbon steels used in knives can have DBTT above room temperature.  

Tempered Martensite Embrittlement

Tempering in the range of 250-400°C (475-750°F) or so leads to a reduction in toughness at room temperature. I wrote about this and why silicon helps prevent the toughness reduction in this article. In short, tempering in that range leads to large plates of cementite (carbides) that lead to a reduction in toughness. 

Tempered Martensite Embrittlement (TME) decreases room temperature toughness but also worsens low temperature toughness. In fact, at sufficiently high temperature the toughness of steel with TME and without is similar. However, the DBTT has been increased because the fracture strength has been reduced by the embrittling cementite plates. 

Knife Steels

Finding toughness vs temperature data for higher carbon steels is very difficult. One good comparison is 4340 and 4140 steel, which are similar except 4340 has a significant nickel addition. The nickel addition gives 4340 quite a low DBTT, well below the temperature at which anyone would be using a knife. The 4140 steel has dropped very little in toughness down to quite low temperatures of -40° or so, meaning 4140 would also be ok for any temperature a knife would likely see.  However, in higher carbon steels the DBTT is already significantly higher, meaning the nickel addition to L6 is quite likely to help with toughness in cold temperature use. 

Knife steels, with their high hardness and many brittle carbides, frequently have a yield strength that is already higher than the fracture strength at room temperature. Perhaps this is why the reduction in toughness at low temperatures is more gradual, because the yield strength is already higher than the fracture strength at room temperature. As mentioned before, low temperature toughness testing of tool steels and martensitic stainless steels is pretty limited.

Studies on Medium and High Carbon Steel

I found only one study [12] that looked at the effect of alloying and tempering on steels with more than 0.60% carbon. They looked at a range of carbon contents from 0.2-0.76C, but unfortunately they tempered no lower than 500°F with the higher carbon steels which is in the tempered martensite embrittlement range. With lower carbon steels they measured a lower DBTT at 300 or 400°F tempering. Here is the effect of tempering on DBTT with steels between 0.33 and 0.41% carbon showing that lower tempering temperatures are better:

Analyzing the full dataset [12] may be useful because they looked at so many variables, despite the use of non-ideal tempering temperatures with higher carbon steels. The studies cited above are a little idealized to show larger trends and effects but this relatively large study can give some feel for how different variables affect DBTT. Carbon and hardness show the strongest effect on the transition temperature. Those two factors are linked, of course. Phosphorous was the next strongest effect after carbon and hardness. Manganese was not found to be a positive unlike the study cited previously. Additions of Cr didn’t seem to change things one way or the other. Molybdenum had what looks like a stronger effect than nickel, at least in the range that was studied. However, there are some high DBTT values for the 3.5% nickel steels which are shifting up the nickel trend line. 

Phosphorous, Nickel, and Molybdenum in High Carbon Steel

Looking at only the 500°F tempering condition does reveal an effect of composition with steels above 0.70% carbon, despite the high likelihood of TME. An addition of nickel improves toughness and lowers the DBTT. Reducing the phosphorous in combination with a molybdenum addition helps toughness even more. The use of lower tempering temperatures and unnotched toughness specimens would lead to even better low temperature toughness.

Update 1/7/2019: I didn’t provide an explanation for why molybdenum helps with low temperature toughness. Molybdenum limits the negative effects of phosphorous by restricting the degree to which phosphorous segregates to grain boundaries [13].

Therefore, even though knife steels are inherently brittle to start with, there are ways to improve low temperature toughness. One way is minimizing the carbon content, especially for a given hardness. Rather than tempering down to a lower hardness, use a lower carbon steel. Using a steel that is at a higher toughness to begin with (higher fracture strength) will also lead to higher low temperature toughness. Buying steel with minimal impurities and nickel additions also help with improving low temperature toughness.

Maximize Toughness with Heat Treatment

Analyzing the above information on what affects low temperature toughness allows us to devise methods for improving it. The first and most obvious is to avoid tempering in the tempered martensite embrittlement range. In other words, don’t temper above 450°F or so. It also helps to use lower austenitizing/hardening temperatures to minimize the grain size and reduce the amount of carbon in solution. You can learn more about how low austenitizing temperatures improves toughness and grain size in this article. It may also be useful to explore multiple quenching to refine the grain size, which you can learn about in this article. 

One Layer Deeper

Why is the yield strength of austenitic steel less affected by temperature? How is it that nickel makes martensitic steel less sensitive to temperature? Time for some dislocation talk. Dislocations are defects in the atomic structure of steel and other materials and the behavior of these defects is key to understanding the mechanical behavior of metals. The image below shows atoms (circles) and the dislocation is the dotted line in between the upside down “T”s. The dislocations are small gaps in the atomic structure. You will see those as black lines in the Youtube videos below. 

I more thoroughly introduced what dislocations are in the grain refinement article. While dislocations are called “defects” this is somewhat of a misnomer as these are not cracks or imperfections of the macroscopic steel. Any steel is filled with dislocations, it is essentially impossible to avoid them, and if somehow they were not present the steel would act much differently. When deformation of steel happens it occurs through the movement of these dislocations. Here is a video of of high resolution microscopy where you can see the dislocations moving throughout a metal:

Since deformation occurs through the movement of dislocations, to strengthen a metal you do it be limiting the movement of dislocations. One way to limit their movement is to introduce more dislocations, because they cannot move through each other. Quenching to form martensite massively increases the number of dislocations which is part of why martensite is so hard. Grain refinement means many more boundaries to impede dislocation motion. Tiny tempering carbides impede dislocation motion which is why high temperature tempering leads to an increase in hardness with high speed steels. 

This is all significant because austenite and martensite/ferrite have a different atomic structure which affects how dislocations move through the material. Austenite is a “face centered cubic” (FCC) atomic structure which refers to how the atoms are arranged. Ferrite is “body centered cubic” (BCC) structure. Martensite is “Body Centered Tetragonal” (BCT) which is similar to BCC but is distorted by carbon atoms trapped in between the iron atoms. 

Effect of Temperature on Dislocations

Dislocations travel along “close packed planes” of atoms which are the planes of atoms which are most tightly packed together. FCC has many close packed planes while BCC and BCT have no “true” close packed planes:

Because martensite doesn’t have a true close packed plane, there is a thermal component to dislocation motion. Some temperature is necessary to feed energy into atomic movement to allow the movement of dislocations. With the close packed planes in austenite the dislocation movement is much less reliant on a thermal contribution. Therefore, as the temperature is reduced the dislocations are less able to move, making deformation more difficult which increases the yield strength of the material. This leads to the high sensitivity of yield strength to temperature. 

Nickel and Manganese

Nickel and manganese additions lead to the promotion of another type of dislocation movement called “cross slip.” Cross slip is the movement of dislocations from one plane to another. This is often observed experimentally with microscopy:

The promotion of cross slip by manganese and nickel means that yield strength is less sensitive to temperature which is what improves toughness and reduces DBTT. 

Summary and Recommendations

Knife steel toughness is temperature sensitive, and can fall steeply at a point called the “ductile to brittle transition temperature” (DBTT). High carbon, high hardness steel has a higher DBTT (lower toughness in the cold) which is significant for knives that will be used at cold temperatures. The DBTT is often above room temperature for knife steels but this is in part due to testing with notched specimens which promotes brittle behavior in an impact test. A reduction in carbon content, grain size, and impurities, and an increase in nickel or manganese, improves cold temperature toughness. Knifemakers that are making knives for cold environments should therefore use steels that are high in toughness, low in impurities, and with nickel additions to ensure the DBTT is low. A steel like 4340 with relatively low carbon and a nickel addition is a good choice though is limited in terms of hardness because of its carbon content. L6 is an option with higher potential hardness but good low temperature toughness data is not available. Low temperature toughness can be maximized through heat treatment by avoiding TME and minimizing grain size and carbon in solution through methods like low austenitizing temperatures or cycling.

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[1] Hoke, J. H., P. G. Mabus, and G. N. Goller. “Mechanical properties of stainless steels at subzero temperatures.” Metal Progress 55 (1949): 643.

[2] Armstrong, T., and L. Warner. “Low-Temperature Transition of Normalized Carbon-Manganese Steels.” In Symposium on Impact Testing. ASTM International, 1956.

[3] https://lne-america.com/testing/lne-charpy

[4] https://www.jeremyjordan.me/ductile-to-brittle-transitions-in-materials/

[5] Morris Jr, J. W. “The influence of grain size on the mechanical properties of steel.” (2001).

[6] http://www.nusatek.com/mechanical-testing/charpy-impact-test.html

[7] Standard, A. S. T. M. “E23-09: Standard Test Method for Notched Bar Impact Testing of Metallic Materials.” Annual Book of ASTM Standards, ASTM, West Conshohocken, PA(2009).

[8] https://www.tf.uni-kiel.de/matwis/amat/iss/kap_9/illustr/s9_1_1.html

[9] https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&NM=250

[10] https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&NM=415

[11] Roberts, G A, and Robert A. Cary. Tool Steels. Beachwood, Ohio: American Society for Metals, 1980.

[12] Schwartzbart, H., and J. P. Sheehan. The effects of carbon, phosphorous, and alloy contents on the notched bar impact properties of quenched and tempered steels. No. IITRI-TR-39. IIT RESEARCH INST CHICAGO IL, 1953.

[13] Wayman, M. L., P. Dumoulin, and M. Guttmann. “On the Role of Molybdenum in Preventing Temper Embrittlement.” Canadian Metallurgical Quarterly 16, no. 1 (1977): 57-60.