The Power of the Coating

Diamonds are pure carbon, and they will weld to the carbon in the steel.

by David Wynn

Director of Technical Services, PMPA

Published December 1, 2024

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Tool coatings are everywhere. Coatings can be found for all sorts of applications. With all the choices out there, it becomes confusing which ones to choose. How do we know the right coating for our application? When we understand tool coatings, we unlock the potential of our machine tools.

Cold Welding
One of the first considerations when evaluating a tool coating for an application involves avoiding cold welding. Cold welding is the process where like materials under extreme pressure are bonded together. The atoms bond and it becomes a homogeneous mass. But why does cold welding matter for tool coatings? The reason is cutting forces in our machines are extreme. When using a tool coating that is similar or the same as the material we are cutting, the tool will weld to the material. This is why there are recommendations for avoiding certain coatings with certain materials.

Cold welding is the reason diamond-coated inserts are not good for steel.

Diamonds are pure carbon, and they will weld to bond with the carbon in the steel.

What do we need our coating to do? Is it to remove heat? To reduce friction? Do we need it to have edge strengthening properties for interrupted cuts? Does it need to help protect the tool from highly abrasive materials that contain aluminum oxides? There are coatings that offer high performance in multiple categories. All-purpose coatings work well in most general applications; however something like AlTiN is not recommended for use in aluminum alloys because of cold welding.

Heat Is the Enemy
The enemy of our tool coating is heat. Specific types of coatings can take distinct levels of heat before they start to degrade. It is important to know the factors that generate heat in the cutting tool. In physics, friction is where the heat is generated. In machining, we say speed causes heat, not feed. I know people are going to say they both cause heat. Both can, but speed puts heat into the tool whereas feed puts heat into the chip. We all know you can feed so hard you break a tool, but the fracture occurred due to forces flexing the tool, not overheating causing coating and edge failure.

Notes to coatings:

Max Working Temp. The temperature the coating will start to break down. You need to maintain cutting temperatures below this value to preserve the coating. Coolant and speed will play a significant role.

Vickers Hardness. The higher this value the more edge protection your tool will have. A higher value provides protection against abrasive inclusions in material. All data in this article is in Vickers Scale HV 0.05.

Coefficient of Friction. Friction is the enemy of our tool coating. Lower values of coefficient of friction allow us to achieve greater speed without coating wear. A lower value will also reduce flank wear because of less pressure when rubbing.

TiN. Titanium Nitride coating has been around a long time. It is that familiar gold color on tooling we have all seen and is still a solid coating for general purpose applications. It has a max working temperature of around 1,000 degrees F. TiN has a Vickers hardness of approximately 2,300 (varies by manufacturer). TiN has a coefficient of friction about 0.40.

AlTiN. Aluminum Titanium Nitride is a great all-around coating. It is a black to gray shiny coating. It has a max working temperature of around 1,400 degrees degrees F. AlTiN has a Vickers hardness of approximately 3,500 (varies by manufacturer). AlTiN has a coefficient of friction about 0.70.

TiB2. Titanium Diboride is great in non-ferrous materials. It is a dull gray-white coating. It has a max working temperature of around 900 degrees F. TiB2 has a Vickers hardness of approximately 2,800 (varies by manufacturer). TiB2 has a coefficient of friction about 0.35.

DLC. Diamond-like carbon (or amorphous diamond) is great in non-ferrous materials. It is a dull black coating. It has a max working temperature of around 750 degrees F. DLC has a Vickers hardness of approximately 8,200 (varies by manufacturer). DLC has a coefficient of friction about 0.1.

Tool coatings are changing all the time. Companies like PH Horn are producing new coatings and reinventing old ones. Experiment with different companies’ coatings within the same families. For instance, one company’s AlTiN may be better than another for your current application. Within the standard families of coatings, there are hundreds of proprietary coatings offered. Stay up to date with your technical sales reps to get coatings that will work best in your application. Having a great relationship with your tech rep will go a long way to getting your job done as efficiently as possible. Here’s to more chips in the hopper and more parts in the pan!

Author

David Wynn is the PMPA Director of Technical Services with over 20 years of experience in the areas of manufacturing, quality, ownership, IT and economics. Email David

Composition of Steel-Reporting Practices for Unspecified Elements

Sometimes the test report shows more chemical elements than we expect. Here’s why.

by Miles Free III

Director of Industry Affairs, PMPA

Published December 1, 2024

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The Certified Test Reports (CTRs), or chem certifications (chemical certs), report the chemical elements and results of other testing of the steel products that we ordered. But in addition to the elements specifically required for a given steel grade, there are small amounts of other elements present. This article will explain the reporting practices and significance (or lack of significance) of these little-understood chemical ingredients or designations.
A steel grade is determined by its chemical composition. Its chemical composition is what makes the development of expected properties and performance possible. Depending on the different chemical elements present, a steel may be designated as a carbon, alloy or stainless steel grade. There are other types of steels — for example, tool steels and heat-resistant steels — but we are focusing on carbon and alloy bar steels most often encountered in our precision machining, production machining shops.
Carbon and alloy bar steels are produced, certified and sold according to a variety of ASTM Standard Specifications. These standards provide not only the requirements for the steels to meet but also the definitions of many terms that describe them and their performance. The three ASTM Standard Specifications that we consulted for this article include ASTM A 29 (latest edition) Standard Specification for General Requirements for Steel Bars, Carbon and Alloy, Hot Wrought; ASTM A 108 (latest edition) Standard Specification for Steel Bar, Carbon and Alloy, Cold Finished; and ASTM A 751 (latest edition): Standard Test Methods, Practices and Terminology for Chemical Analysis of Steel Products. These are the foundations for defining commercial product compliance and product attributes.

Elements by any Other Name: Specified Elements
Each grade of steel has a list of chemical element ingredients that must be present for the material to be compliant and meet specification; these are called specified elements. Specified elements are present within a specified minimum and maximum range, as listed in the specification. Carbon, manganese, phosphorus and sulfur are specified elements in carbon steels. Lead can be a specified element if the steel is a leaded carbon steel. Note that due to differences in testing and analytical procedures, these minima and maxima can vary based on whether the testing is at time of heat or cast (melt) analysis, semi-finished or in the final product. Due to segregation in different types of steels, variations in some of these elements may not be cause for rejection based on product analysis. In alloy steels, alloying elements including nickel, chromium and molybdenum are common, with lead, vanadium and boron also possibly specified.
There are other chemical elements present that may or may not be reported on the certification documents. In casual conversations, these names are often substituted for one another, but each of these categories of additional chemical make up can have a different origin and, perhaps, impact. ASTM A 751: Standard Test Methods, Practices and Terminology for Chemical Analysis of Steel Products authorizes the producer to report unspecified as well as specified elements on the test report. This standard defines specified elements, intentionally added unspecified elements, residual elements and trace elements. Not listed, but often used as a name for some of these categories are the terms “incidental elements” and “tramp elements.”

Elements by any Other Name: Unspecified Elements
Unspecified elements may be either intentionally added, or unintentionally find their way into the melt as component of the raw materials or process. Intentionally added (in controlled amounts) unspecified elements can include deoxidizers, grain refiners or deliberate, modest additions of other elements to enhance or develop properties. Silicon is an unspecified element (not listed in Table 1 of ASTM A29) as an ingredient but can be added in ranges: 0.10% max; 0.10%-0.20%; 0.15%-0.35%; 0.20%-0.40% or 0.30% to 0.60% (per footnote B). Sometimes the amount of chromium added to a medium carbon, plain carbon steel is increased just to ensure achievement of a minimum hardenability or mechanical property for the customer.
Unspecified elements that are not intentionally added may be called residual elements or trace elements. Residual elements are picked up from either initial raw materials (in the scrap, ores or fluxes) or can be absorbed by the melt from refractories. In electric furnace steels, copper is a prime example. Copper is contained in small quantities in the scrap steel charged into the furnace, showing up as higher quantities in electric furnace melt steels compared to basic oxygen steels, which have a limited scrap charge. However, the electric furnace shop can add quantities of direct reduced iron (DRI) to reduce the level of residuals in the steel by dilution. Occasionally, copper can be specified as a 0.20% minimum.
Trace element is the designation given to residual elements that appear in very low concentrations — less than 0.01% by weight.
According to the Steel Bar Product Manual, Iron and Steel Society, 1994, “Certain elements are present in small quantities in most steels. They are not intentionally added but have been retained from the raw materials used during production of the steel. These elements are considered incidental or residual as long as they do not exceed certain
maximum limits.”

What’s Left?
Sometimes, a test report will show a percentage “all others” or “balance.” Typically in steels that value is referring to iron, which makes up the balance of the material.

Conclusion
Zero is a charming artifact of the 20th century, back when there were only three black-and-white TV channels, and when parts-per-million precision required government laboratory levels of equipment and budget. Parts per billion were not even a thought. As analytical methods have advanced, the zero has receded so far that in some commercially common laboratory methods, we can truly “count atoms.” Many chemical elements that are naturally occurring can be found in residual, incidental or trace levels or quantities, given a suitably equipped laboratory and budget. They can be found as well in foods, soils, drinks and packaging, as well as in the metallic and polymeric raw materials we machine in our shops.
The reporting of chemical composition is important, but understanding the levels of the chemical elements in terms of weight percent and specification threshold is critical if we are to avoid overreaction at seeing some trace or incidental value as cause for alarm.

Author

Miles Free III is the PMPA Director of Industry Affairs with over 50 years of experience in the areas of manufacturing, quality and steelmaking. Miles’ podcast is at pmpa.org/podcast. Email Miles