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Research like this is made possible by Patreon supporters. That allowed me to purchase the carbide straightening hammers and perform the experiments below. Join at Patreon.com/KnifeSteelNerds if you would like to support further research.
Video
There is also a video version of the following information:
Knife Engineering 2nd Edition
The following study was included in my new book Knife Engineering 2nd Edition. Those that bought the book have already seen some of this information. There are several other studies in the book that have not been published on my website. Go buy the book to see them!
Peening
Using a carbide hammer on a piece of steel is a type of “peening.” Peening is a general term for working the surface of a material, usually by cold work. I have an older article about cold work such as cold rolling and cold forging. And a somewhat newer article where we tested the properties of 52100 after cold working. Cold work strengthens/hardens a metal or material. If you cold work an entire piece such as through cold rolling you get an increase in hardness depending on the percentage of cold work, such as we measured with 52100:
Of course if you cold work too much eventually you will introduce cracks, just like if you continue forging a piece of steel when it has gotten too cold. Peening is a form of cold work that is only applied to the surface. Perhaps the most common and best known form of peening is “shot peening” where steel shot is sprayed against the metal to cold work the surface. This is sometimes done to give a matte finish to metals but generally it is performed to enhance performance of certain parts. The cold work applied to the surface creates an “residual compressive stress.”
Image from [1]
Image from [2]
When knifemakers hear “residual stresses” they sometimes worry but these compressive stresses are not a bad thing. Residual stresses are those stresses that remain when the original force is removed, the stress remains, i.e.. “residual.” Shot peening creates a “compressive” residual stress, as opposed to tensile stress. When the surface is in compression that creates an environment where crack growth is suppressed. You can think of the compressive stress as “pushing” cracks back together rather than pulling the cracks open. So shot peening is often performed on parts that fail to “high cycle fatigue,” or cyclic loading. These are parts like crankshafts, gears, etc. that see millions of cycles and fail due to slow crack growth. Shot peened parts last through many more cycles than those that are not peened. Knives do not generally fail due to high cycle fatigue so this wouldn’t necessarily lead to improvements in knife performance. But we would not expect it to negatively affect performance either.
Image from [2]
However, there can be dangers to peening. Just as a metal can be cold worked so much that it cracks, a part can also be “overpeened” leading to surface cracks, as seen below in 1045 steel:
Image from [1]
Peen Straightening
Straightening pieces by peening has been around for over 100 years. For example, here is one illustration of how to do it from 1910:
Image from [3]
The peening is performed on the opposite side from typical forging. The peening “stretches” the worked surface which returns the piece flat. This can be done with most peening methods including shot peening. However, hammer peening has become common for knifemakers over the past decade or two. This can be performed with a carbide ball or with a chisel.
Image of carbide ball hammer and chisel from Kyle Daily
The chisel and carbide ball lead to somewhat different behavior when it comes to stretching the material. The chisel leads to more “vertical” stretching while the ball leads to even stretching around each impact:
Schematic illustration of the movement of material around a carbide ball impact on the knife. Image from Kyle Daily.
Schematic illustration of the movement of material around a chisel impact on the knife. Image from Kyle Daily.
The chisel is preferential in some ways because generally for straightening you only want material moving in the direction of the curve you are trying to straighten. However, the stress from a chisel can be more dangerous because of how concentrated it is, potentially leading to overpeening. Kyle Daily recommends using a carbide ball hammer (which he makes and sells) and he uses a method where he makes vertical lines of the carbide ball impacts to create the directionality instead.
Image of straightening a knife from Kyle Daily.
Kyle has a YouTube video where he shows his method:
Tests with Peening
I have gotten many questions from knifemakers the past few years asking if straightening by peening is leading to microcracks that are embrittling the steel. As described above, peening, does not mean cracks are forming. It is a cold work process, and as long as the steel is not overly cold-worked there are no negative effects on properties. To test this, I used a carbide ball straightening hammer on some toughness coupons of heat treated MagnaCut. For one condition, I hit the pieces in many places before grinding and finishing both sides (the peened surface was removed). On the other coupon I ground and finished the coupons first and then I hit it many times with the straightening hammer after. With these three different conditions (with three samples of each condition) I measured the impact toughness. This test measures the energy required for breaking the steel. So if there were any pre-existing cracks we would expect to see a reduction in the measured impact toughness. The results in comparison with samples that saw no carbide hammer blows are shown below:
All of the specimens performed very similarly to each other. The one that was straightened after finishing did have slightly higher toughness. I am not sure if this is due to improvement from the peening or if it is just from statistical variability from testing. It isn’t impossible that the surface peening did help a little bit but the improvement was small in either case.
Dangers of Peen Straightening
While my tests showed no negative effect of hammer peening, there are still dangers to the practice. A knifemaker previously sent me a knife that had a crack in the handle. When the handle scales were removed it was found that the crack had initiated within the peening dimples:
It is a bit hard to tell in the pictures but these were relatively deep peening marks. It appears that this steel was “over peened” and thus cracks were formed in the surface of the steel and grew from there.
Summary
Peening is a surface cold working process to create a residual compressive stress in the surface. A compressive residual stress can be beneficial, especially in parts that fail to high cycle fatigue (knives do not generally fail in this way). Peening is an effective way to straighten thin curved pieces of metal, such as warped steel knives. Peening does not negatively affect steel when done properly. However, cracks can be formed if the steel is “over peened.”
[1] Maleki, Erfan, Okan Unal, and Auezhan Amanov. “Novel experimental methods for the determination of the boundaries between conventional, severe and over shot peening processes.” Surfaces and Interfaces 13 (2018): 233-254.
[2] https://www.amtechinternational.com/shot-peening-benefits-gears/
[3] Smith, Robert Henry. Text-book of advanced machine work: prepared for students in technical, manual training, and trade schools, and for the apprentice and the machinist in the shop. Industrial education book Company, 1910.
