Diffusion Barrier Coatings for Graphite Fixtures

    Vacuum heat treating shops commonly use graphite fixtures on which to set parts to be treated.  Graphite fixtures have excellent thermal stability, allowing them to be used again and again through multiple heat treat cycles. In addition to that, graphite does offer significant cost advantages, when compared to other materials used to make fixtures (high nickel/chromium or moly alloy type fixtures)

Graphite Fixtures and Eutectic Bonding

Eutectic melting is the process where an alloy of two or more metals, when heated, will completely change from solid to liquid at the same temperature. There are processes in which the eutectic melting is a desired result, such as brazing. There are thousands of eutectic compositions used in brazing applications. They can play a major role when heating a brazing filler metal up to the brazing temperature, as well as when cooling after brazing. During heat-up and melting, the eutectic composition will/should be the first to melt, and because of its narrow melting range (solids and liquids are the same temperature), it will flow into the braze joint.



Graphite Compressive Strength vs. Graphite Flexural Strength

   Flexural Strength with most graphite material is on average only 50% to 60% of its Compressive Strength.
            By definition, Compressive Strength is the capacity of a material to withstand loads tending to reduce its size. Measured, it is the value obtained by plotting applied force against deformation. At compressive strength limit, the material will fracture, or deform irreversibly.
             Flexural Strength is defined as the ability to resist deformation under load. It is measured with the help of The Transverse Bending Test. In this test, a sample is bent until it fractures, using a 3 point flex test technique.  Flexural Strength is measured at the highest stress experienced within the material at the moment of rupture. These specifications apply to graphite materials and are commonly found on material specification sheets.
           Confusion can occur, if the definitions are not correctly read, interpreted or understood.  Both parameters refer to the ability of a material, to resist deformation (withstand loads).  The difference between the two parameters is due to the methodology applied to calculate the loads.   Compressive Strength is measured when force is generally applied uniformly on one surface, while the opposite face is fully supported.
Flexural Strength is measured as a bend test. The material is supported on 2 points placed at the edges of the material, while force is applied in the center of it. Intuitively, the Compressive Strength of a material will be greater than the Flexural Strength of the same material.
            We had an experience that demonstrated how the wrong interpretation of these materials specifications resulted in component failure.  We received a customer call concerning an application using graphite plates in a sintering process.  The customer informed us that some of the graphite plates had fractured during use.  We have produced the plates working closely with the customer, based on their process requirements.  Additionally, these plates have been used for years by the customer with no failures.
            Their system consisted of one graphite plate used as a base, fully supported by the bottom of the furnace, a stack of parts to be sintered, and finally a top graphite plate.  The graphite plates, allowed for a threaded metal rod secured to the bottom of the furnace, to penetrate the center the assembly.   A large round nut was used to secure everything in place, and apply the necessary pressure.  This design resulted in an assembly that was subjected to mainly compressive forces throughout. The graphite grade was more than capable to withstand the compressive forces applied within the system as originally designed.
            When attempting to resolve the fracturing issue with the customer, we looked into any possible changes within the process.  We discovered that the originally designed part, which was virtually a solid disk, has been re-designed by the customer for weight savings, into a ring, with a large I.D.  Our team realized that the top lid was no longer supported.  Once force was applied to the center of the lid, the material fractured.
In this instance, the lid was no longer experiencing compressive strengths, but in fact it was experiencing flexural strengths. This resulted in reducing the maximum force the lid could withstand by approximately 50%.  With this design change, the original material was no longer suitable in this application.  The solution was relatively simple. Higher strength graphite material was used to replace the original graphite grade.
            In conclusion, while the process did not chance, the different geometry of the parts caused a switch from compressive force, to flexural force.  The updated lids with adequate flexural strength were tested, and ran flawlessly. The lids are currently in full production and performing as expected.

Explaining The Coefficient of Thermal Expansion in Graphite Materials

At Semco Carbon, we are constantly thinking of new, relevant and useful topics to incorporate into our periodic blogs. 

Graphite Degassing System

        When discussing materials used in Aluminum Foundries, graphite occupies an important spot, especially when referring to aluminum smelting operations. 

What is Graphite?

What Is Graphite? 

Is Machining Graphite Plates, Graphite Rods Really for You ?

    In many instances we receive requests for basic shapes of graphite. Most of the time, the consumer is looking for graphite rods, or graphite plates. The intent is to machine these basic shapes into finished product. Although, we are service/solutions provider, we are happy to supply our customers with raw graphite materials too.  

Graphite Molds and Continuous Casting

    Continuous casting is a casting method used in high volume production of metals with a constant cross-section. The method uses an open ended graphite die which is surrounded by a copper jacket. Through which is poured molten metal. The graphite mold is typically water cooled. This allows the molten metal to solidify within the die, the custom solid metal form is then extracted from the mold and pulled/passed through rollers and water sprays. This process removes the heat from the metal, and gradually solidifies it.  

Graphite Machining Potential Pitfalls

So, you are the proud owner of a machine shop. You have a customer that is in need of graphite machined parts, and you want to make your customer happy.  Right?  So you take the job but quickly figure out that it could be more complicated to produce these parts than you originally thought it would be. Initially, you need the correct raw graphite material, but when you search for sources, you find out that there are thousands of graphite grades out there, and very little help in finding out what material is really needed for your project. Your client is no help there either, and then you do some research and learn that your typical machining process, using liquid coolant won't be able to do the job.  Running the job dry will leave a black dusty mess inside your well maintained CNC equipment and possibly void the warranty.