Tool life can vary when machining carbon and alloy steels despite the use of our best technology and our efforts to control our processes.
Here are 6 factors that can affect tool life in your shop.
Variations between suppliers. Suppliers’ melt processes, scrap practices, melt recipes, and reduction in cold drawing and straightening practice can significantly affect the way that the chip breaks and resulting built up edge on tool and resulting surface finish. Even though the grade is the “same.”
Variations in Chemistry. A potential subset of variations between suppliers, the fact is that a plain carbon grade with 0.005 wt. % Sulfur will not machine at all like the material with 0.025 wt% sulfur.
Variations in grain size. While this factor is typically more relevant in stainless steels, when machining forgings, blocky structures resulting from excessive forging temperatures can result in inconsistent machining performance.
Variations in microstructure. In this case, it is not so much about the grain size, as it is about the structure present. This is particularly problematic in the ~0.40 wt% carbon alloy grades like 4037.
Decarburization or scaling on the work surface. Decarb can result is a carbon poor gummy surface, only to then transition into a fully carbon containing microstructure. Scale on the work surface can result in excessive tool wear, due to the very high hardnesses of the various iron oxides that may be present (Hematite, the red oxide of iron, Fe2O3, has a microhardness of approximately 1030 DPH.
Deoxidation /High Inclusion Count. Free machining grades such as 12XX and 11XX steels are typically sold to a “Coarse Grain Practice” with no deliberate additions of grain refiners or deoxidizers. Sometimes, you may find deliberate additions of Silicon to 1144 in order to improve the internal soundness of the steel, the resulting silicates can abrade the edge of the tool when running at the surface feeds expected for a resulfurized steel. The addition of aluminum as a grain refiner can cause rapid edge wear as well. Rarely, very rarely, one might encounter exogenous inclusions entrapped in the steel from melt and casting. These can be real showstoppers.
Takeaway:Purchasing the same item from different suppliers hurls the full range of global variation at your machining operations. Standardizing on a single supplier for an item will allow you to get to steady state in your process.
Photo courtesy of via CTE Magazine Plus
Here is a post to print out and put in your tool box for reference if you work in the shop…
It’s the machinists in the shop that get a first sense of “Something is Different” or “Something Changed” when a new batch of material is put into production. In my experience, these are the five things to check to confirm that it is a purchasing/ supplier issue rather than the machining process out in the shop.
Different Than Ordered
For Machining- Significantly Different Sulfur
Different Supplier-Different suppliers can have different processes, recipes, standards, and practices. Changing or mixing suppliers can induce more variability in a process than an out of control operator in the shop. Different Process-Maybe it is the same supplier- company, or location. but perhaps they can make the product on two or more different process lines. maybe one from a coil to bar, the other from bar to bar. Maybe the straightners are different. Different Than You Ordered-Maybe you ordered Cold Drawn and they supplied you with Cold Finished. In Cold Drawing, the cold work of die drawing adds strain which changes the material’s mechanical properties. Cold Finished could include cold drawing, but often means hot roll that has been turned and polished to get the same tolerance and surface finish, but does nothing to change the material’s mechanical properties. Different Deoxidation-Maybe you ordered aluminum fine grain, and they supplied Vanadium. Maybe you wanted Silicon .10 max coarse grain practice, and the only steel in stock was .15-.35 silicon killed and your purchasing department said, “Yeah, we’ll take it.” Different deoxidation can affect mechanical properties too. For Machining-Significantly Different Sulfur Levels-There are several “tipping points” for sulfur in the non- free-machining grades of carbon and alloy steels. Steels showing a Sulfur analysis of 0.020% minimum never were submitted to me for machining claims in over 20 years of managing labs and claims in my steel mill career. Steels below 0.018% would often be sent back, especially in plain medium carbon grades like 1045 and 1050. Typically the customer would complain about fighting it to get production.
Steels below 0.015% Sulfur would almost always elicit a very heightened urgency from the customer who couldn’t keep tools in the machine and couldn’t get finish. Steels below 0.010% almost always got a call from their VP to our VP who then put me on the case. This is especially true in medium carbon alloy grades (4140 etc.) where the material is challenging anyhow, especially on screw machines.
I had one claim once upon a time for a batch of 4140 with 0.003% Sulfur. The guys in the shop called it “Toolproof.”
So these are five quick items to check on the material certs to see of the unexpected variability you are encountering in the shop might, just might, be the result of supply chain changes upstream of your screw machines. (Note: My experience was primarily supplying the shops running high volumes on cam automatic screw machines. Even with carbides, coatings and cnc controls, the 5 items listed above can make a serious difference in your shop’s performance.)