Once you decide to manufacture any product, you’re often faced with the monumental challenge of selecting a material. When you’re ready to move onto full production after prototyping, you need 100% certainty that the material you have ordered is completely certified, and that it matches all your expected requirements. This includes amongst others: its mechanical strength, chemical composition and overall quality. This is when a Material Certificate becomes a critical factor. Talk to your CNC machinist about obtaining these certificates.
What is a Material Certificate or Mill Test Report (MTR)?
A material certificate is called many things in industry. For the purpose of this article, we will refer to it as a ‘mill test report’ (MTR). An MTR serves the purpose of displaying all the mechanical and chemical properties of the specific material that you purchased. This information is supplied to prove that the material you have bought conforms to an international standard like ASTM (American Society for Testing of Materials). In most cases you will need to specifically request this report. The main purpose of the MTR is quality assurance.
What Information does an MTR Contain?
You will normally find the following information in an MTR:
The manufacturer of the material is indicated for traceability. Material supplied from a well-known supplier can go a long way in ensuring quality.
A general description of the material is indicated. This is to ensure that the MTR you were supplied with matches your actual material.
The total, or per unit weight of your material is listed. As the previous point, this serves as another check to make sure you have the correct MTR.
Metals come in many different grades. For example, 304 Stainless steel contains Nickel and Cobalt, whereas 316 SS contains Nickel, Cobalt and Molybdenum. You must make sure you are using the correct grade. It is impossible to differentiate between different grades of steel via visual inspection.
Material Heat Number
This is basically just an identification number that can trace the material all the way back to the furnace it came from. The number usually contains the furnace number, year it was made and the melt number. This number can usually be found stamped into the material. This number will connect the MTR to your actual material.
The chemical composition of a material determines its grade, this allows you to look up the required standard for a specific grade to determine if your material meets the specification. If you want to make sure that your chemical composition is indeed within a specific standard you can use these values to compare.
Mechanical Test Result
The results of a few standard tests are indicated here. The tests can include tensile tests that are performed to a pre-defined standard. This allows you to compare the results of your sample with the industry accepted standard.
This indicates under which international standard the material was tested. This can be ASME (American standard of mechanical engineers) or ASTM (American society for testing of materials). This ensures that the material passes stringent international requirements.
The MTR needs to be certified by the mill. This is usually in the form of a signature from a responsible person at the mill. This makes the document binding and it will be easier to act on incorrectly supplied material.
When is an MTR Important?
There are numerous reasons why you would want an MTR. If you are manufacturing a component that could present a major risk if it fails, such as a shaft or a pressure vessel then you will need to make sure that the material specifications you used to design the part are the same as your actual material being used in manufacturing.
If you are marketing a product as being a specific material, then you will need an MTR from your supplier to prove that you are using the stated materials. Someone might use your part thinking it is made of high strength duplex stainless steel, but it is in fact standard stainless steel. This could result in unexpected failure and possible injury. Even if your component is just a fidget spinner, you might want an MTR to ensure that you are getting the material you paid for.
If you are making parts for aerospace or automotive clients, then it is best to confirm with your customer what their requirements are. If they are using a specific quality protocol such as PPAP (Product Part Approval Process) then different levels would have different requirements. For example, level 1 does not need any material documentation, whereas level 3 would require a product sample and complete supporting documentation which includes material certification documentation.
If your part is critical then you should always request an MTR from your CNC Machining company. Even if it is not requested by your client, the responsibility of using the correct material ultimately lies with you as the product designer. You might be able to shift the blame to your supplier, but it is your brand and image that will take a knock if your product fails due to sub standards materials. An MTR dramatically reduces the unknowns in your product, and as a result reduces your risk.
James Abbott is the Managing Director at Challenge Engineering, a repetitive engineering company based in Sydney Australia. Give him a call if you want to know more on (02) 9632 0010 or email firstname.lastname@example.org