Stainless steel is one of the few common construction metals that hold up in harsh conditions. Condensation, rain, or even splashes of salt water or acids – many stainless steels can take on all of this. When looking for a widely available and relatively affordable high-strength material for wet, outdoors or marine environments, the austenitic AISI/SAE 304 and 316L stainless steels are probably at the top of your list.
Yet for the workshop builder, stainless steel has one annoying property: it is difficult to cut and drill. As anyone who has been working with stainless steel for some time can tell you, drilling stainless steel all too often ends in dull or broken bits, jams and unfinished holes.
So why, exactly, is stainless steel so hard to drill? And how should you properly go about it?
I am going to answer these questions in this series of three articles on drilling stainless steel. The answer is divided into three articles as follows:
- In this article, the first in the series, we are going to try to understand what makes stainless steels hard to drill.
- If you are more interested in how to drill stainless steel properly, you can jump ahead to the next part, which tackles this practical issue directly.
- You may also want the check the last part, which gives more specific instructions on drilling some common stainless steel products and special stainless alloys.
The information should be both interesting and useful, and I hope it may help you in your work. Next, we will start by dispelling a myth.
It’s not the hardness
One common explanation why stainless steel is difficult to drill is that it is just a very hard and strong material. And true enough, hard materials are hard to drill, so this explanation seems to make sense.
The only problem is that, in its most common austenitic alloys, stainless steel is not particularly hard or strong. You can verify this by comparing its hardness or tensile strength to other common steels using material property databases or materials science handbooks.
The table below makes this point concrete by comparing the hardness and strength of two common stainless steel alloys – the AISI/SAE 304 and 316(L) – to common plain steels. As you can see, both of these stainless alloys are not really any harder than the low-carbon SAE 1020 or the medium-carbon SAE 1040 steel.
|low-carbon||SAE 1020||30 (210)||55 (380)||111 HB||hot-rolled|
|medium-carbon||SAE 1040||42 (290)||76 (520)||149 HB||hot-rolled|
|austenitic stainless||SAE 304||30 (205)||75 (515)||123 HB||annealed|
|SAE 316||42 (290)||84 (580)||150 HB||annealed|
Comparing the different grades more closely, the SAE 304 and a low-carbon plain steel SAE 1020 have very similar hardness and yield strength in their most common delivery conditions. They also do not differ dramatically in tensile strength. The difference between SAE 316 and a medium-carbon plain steel SAE 1040, on the other hand, is even smaller, up to the point of being negligible.
As most other low-to-medium carbon plain steels, neither SAE 1020 nor 1040 are particularly difficult to drill. Looking just at the strength and hardness, you would not expect the stainless 304 and 316 alloys to be too hard to drill into either. And yet they are. But then, this is obviously not due to their hardness.
So if it is not the hardness, what is it about stainless steel that makes it difficult to cut?
We will next take a look at three interconnected factors that really are the ones behind the drilling difficulties: toughness, work-hardening and heat generation.
Factor 1: Toughness
Stainless steels are not hard, but they are tough.
Although these two words mean almost the same in vernacular, they have related yet distinct meanings in materials science. Whereas hardness refers to a materials capability to resist indentation, toughness refers to its capacity to absorb energy in deformation.
Materials with high toughness are called tough, and are hard to cut or break because of the energy absorption: deforming the material takes a lot of work. Common tough materials are mild steels, most pure metals like copper and gold and rubbers.
Materials with low toughness are brittle, and take only little energy to break. The obvious everyday example here is glass, which, as everybody knows, is all too easily shattered to pieces.
So, how tough is stainless steel? There are many ways to answer this question. Firstly, scientists and engineers have developed many different metrics for toughness, the most important of which are toughness, fracture toughness, and notch toughness.
Second, there are measures of a related material property called ductility which can be combined with strength data to estimate toughness. These data are simpler to measure as a part of a tensile test and more widely available than the proper toughness data, so we will do the comparison based on ductility instead.
Toughness of stainless steel
The table below compares the SAE 304 and 316(L) stainless steels to two common plain steels: a low-carbon SAE 1020 and medium-carbon SAE 1040 alloy. In the hot-rolled condition, these plain steels have roughly the same strength properties as the stainless 304 and 316 alloys. The 1020 and 1040 steels are also relatively ductile, with minimum percent elongation of 25% and 18%, respectively (Callister&Rethwisch, 2011).
But as the table shows, the stainless steels win out these plain steels in ductility by a wide margin, and may stretch up to 70% in the tensile test. With strength properties roughly the same, the greater ductility of the stainless steels translates directly to greater toughness.
|austenitic stainless||SAE 304||40…70||annealed|
Factor 2: Work-hardening
The toughness of stainless steels helps at least partly to understand why they are difficult to drill: they can absorb a lot of energy in stretching before they fail.
Still, the connection to drilling remains a bit unclear: what does this toughness really mean for the drill bit?
To understand this, we have to look at what happens in a tough material during deformation. When a tough and ductile material is plastically deformed, it gets progressively harder and harder. This hardening process is called strain or work hardening, and occurs both in the neck of a tensile test specimen as in front of the drill bit cutting edge. The hardening continues so that by the time of the final fracture, the material strength in the failing section may be many times the yield strength.
This work hardening process is the main reason behind the difficulty of drilling into stainless steel. As an exceptionally tough and ductile material, stainless steel has an incredible capacity to work-harden. Through work hardening or “cold work”, as intentional work hardening is often called, the common SAE 304 and 316 alloys can increase their strength from the modest yield point at around 200 MPa all the way up to 1500 MPa – a seven-fold increase!
Returning to drilling, we now see that the moderate yield strength and hardness values of stainless steel are only the starting point. As your drill bit is cutting, the material resistance early on in the chipping process is low. However, as the cut proceeds and the material in the chip and on the cut surfaces is deformed, the material progressively work-hardens, so that by the time a given chip has been released the resistance has increased dramatically.
The aggressive work-hardening and high final strength of stainless steels leads to high cutting pressures and resistance. Indeed, the specific cutting force – a quantity used in machining to predict tool loads and power requirements – is above 2000 MPa for practically all stainless steels (see e.g. this article in the Sandvik Knowledge Center).
Fighting this resistance takes a hard-wearing drill bit and a powerful drill driver.
Still, this is not yet the full story. We have ignored one of the major variables in cutting tools: temperature.
Factor 3: Heat generation
Even after seeing the high final strength of stainless steels after work hardening, you may wonder what is the big deal: with hardnesses above 60 HRC, your regular high-speed steel (HSS) drill bit is still much harder than even the full hard stainless steel, and your drill press not lacking in torque.
Here comes the real kicker: due to the toughness of the material, cutting stainless steel generates a lot of heat. This heat is generated in the friction between the drill bit, the body material and the chip as well as in the deforming steel itself. Although most of the heat is fortunately generated in the chip, some of it goes to the tool, which consequently heats up to a high temperature. The situation is still made worse by the the low heat conductivity of stainless steels, which slows the diffusion of the frictional heat from the tool cutting face into the the chip, and further raises the tool temperature compared to other materials.
So the question to ask is not whether your drill bit is hard enough for stainless steel at room temperature, but rather if it is still hard enough at a much, much higher temperature.
How high the temperature near the drill bit cutting edges actually rises will depend on many factors, such as speeds, feeds, cutting angles, whether coolant or lubricant is used, and so on. However, unless choices are made specifically to keep the heat generation down, the bit may have to face temperatures of 1000 F (550 C), 1400 F (750 C) or even 1600 F (900 C) (Hui-Bo et al, 2017).
This is bad news for your drill bit. All tool materials, however advanced, lose hardness with increasing temperature: their hot-hardness or red-hardness is much lower than the room temperature value.
Let us take the most common drill bit material – high-speed steel (HSS) – as an example. The standard non-cobalt-based high-speed steel types such as M1 and M2 start out at room temperature with a respectable hardness of around 65 HRC, varying a bit by the type. This hardness is certainly enough for cutting stainless steel.
However, already at a low cutting temperature of 500 F (250 C), the hardness is reduced to 60 HRC, and by 800 F (425 C) to 55 HRC (Bayer&Becherer, 1989). At this hardness level, the bit can probably still resist the 2000…2400 MPa cutting pressures of 304 and 316L stainless steels, but already suffers from a higher wear rate.
As temperature increases above 900 F (500 C), the HSS drill bit starts to be in serious trouble. By around 1000 F (550 C), its hardness has decreased to only 50 HRC, which is already critically low for stainless steel. Above 1000 F, it gets only worse, as the HSS hardness enters a nosedive: 45 HRC by 1100 F, 35 HRC at 1200 F, and a mere 10 HRC at 1300 F, so that by 1400 F, the bit no longer has useful hardness for cutting stainless steel – or any other metal for that matter.
There are of course more advanced tool materials offering hot-hardnesses superior to those of standard high-speed steel, which may allow you to cut stainless steels at slightly higher speeds. Cobalt-based high-speed steel alloys retain a relatively safe hardness of 55 HRC up to 1000 F (550 C) and solid tungsten carbide bits up to 1300 F (700 C).
However, none of the common drill bit materials will easily take on 1500 F (800 C), let alone higher temperatures. Heat generation is a major problem in drilling stainless steel, and cutting temperature is obviously something to be considered.
In this article, we learned that stainless steels are difficult to drill because they are tough, work-harden in the cutting process, cause high cutting pressures and heat up the tools.
I hope this brief glimpse into the challenges of drilling stainless steel has dispel some of the myths surrounding the topic, and helped you understand the real reasons behind the drilling difficulty.
But then what? Can you still drill austenitic stainless steels, and how? What about other types of stainless?
These are questions that we are going to answer in detail in the next parts of this series. If you are interested not only in understanding but also doing, be sure to continue to the next article!
Bayer, A. M., Becherer, B. A. (1989) “High-Speed Tool Steels”. In: ASM, 1989: ASM Handbook, Vol 16: Machining
Callister, W. D., Jr & Rethwisch, D. G. (2015). Materials science and engineering (9th ed., SI version.). Hoboken, NJ: Wiley.
Hui-Bo, H., Hua-Ying, L., Yang, J., Xian-Yin, Z., Qi-Bin, Y. & Jiang, X. (2017). A study on major factors influencing dry cutting temperature of AISI 304 stainless steel. International Journal of Precision Engineering and Manufacturing, 18(10), pp. 1387-1392. doi:10.1007/s12541-017-0165-6