Of all the materials that mankind has ever discovered or developed, my extremely biased opinion is that steel is the greatest of them all. That being said, to say steel is the greatest material of all time is like saying that cake is the greatest food of all time. There are a wide variety of different types of cake that have different flavors, different ingredients and are made for different occasions. You can apply that same logic to steel. One of the things that makes steel such an amazing material is its versatility. Different types of steel can be used to fit all sorts of different applications. Whether the application be in heat -resistance, corrosion-resistance, heavy-tension or compression, lots of impact, wear-resistance, or some combination of these, there is a good chance that there is a steel out there that can fit that application. A while back, we did a blog on stainless steel for heat resistance, and we have decided that we are going to take that concept and carry it on to other types of materials in the Temperform portfolio. The hope with this series is that we will be able to give an overview of the different types of materials that Temperform offers to their customers. This can assist in buyers of castings, and others, making decisions on the right material for their application. In this series, we will be characterizing the materials from a performance perspective, a castability perspective, a heat-treatment perspective (when applicable), and also including some fun facts about the material.
Various steel made at Temperform
Austenitic (Hadfield) manganese steel
Though we unofficially kicked off this series with “Stainless Steel for Heat Resistance,” we will be officially kicking off this series in this blog with one of my personal favorite steel types, austenitic (Hadfield) manganese steel. Though this is not even close to the most commonly poured material at Temperform, it is one of the most unique and intriguing materials ever developed from a material science and engineering perspective. The history of manganese steel dates back to the 19th century, when Sir Robert Hadfield discovered/developed the original manganese steel alloy, which contained 1.2% carbon and 12.5% manganese. Hadfield further found that heat treating the material to approximately 1000° Celsius, followed by a water quench enhanced the toughness of the material considerably. Hadfield patented manganese steel in 1883 and he was knighted in 1908. Today, the term “Hadfield manganese steel” is not the official designation, it is now called “austenitic manganese steel” and the material is largely covered by the specification ASTM A-128.
What makes it austenitic manganese steel?
To be considered austenitic manganese steel, the steel must have a carbon content of at least 0.7% and a manganese content of at least 10%. Generally, the ratio of manganese to carbon is 10-1, though that can change as the alloying content is increased. By technicality, ASTM A-128 only covers a manganese content of up to 14% and a carbon content up to 1.45%, though there have been grades developed with higher manganese and carbon contents (Temperform offers one grade that contains 20% manganese). There are also some grades of manganese steel that contain other alloying elements for a variety of different reasons, those typically used are chromium, molybdenum, and nickel.
Performance and applications of manganese steel
The best way to characterize manganese steel in terms of its material properties would be to call it a low to medium strength, high ductility, and super tough material. Manganese steel is also non-magnetic. The two most unique properties of manganese steel are the ability of the material to work harden, and its extremely high toughness.
Work hardening in manganese steel
Work hardening is the phenomenon that occurs when manganese steel is exposed to impact on the surface of the metal. This impact that occurs on the surface of the metal actually causes the material to gain hardness. The more it’s impacted, the harder it gets. As supplied, manganese steel typically has a hardness of somewhere around 220 BHN (Brinell hardness number), but it can work harden up to as high as 500 BHN. The mechanisms of how work hardening occurs are not well understood at this time, though they are being further explored. One fun fact about manganese steel is that some casting suppliers (not Temperform) can and will supply manganese steel in the pre-work hardened state. To supply the castings in the pre-work hardened state, they are usually covered with explosives, which are then detonated to produce an impact on the surface of the casting.
High toughness in manganese steel
The high toughness in manganese steel is linked with the high ductility. Most manganese steels will have somewhere between 20-40% elongation, and room temperature impact energy on a Charpy V Notch up to 100 ft/lbs in the heat-treated condition. The toughness in the material comes from the transformation of the microstructure during heat treatment. As-cast, manganese steel has a predominantly austenitic structure with grain boundary carbides, which makes the material very brittle. The heat treatment cycle for manganese steel (which we will discuss more later) dissolves those brittle carbides at a high temperature, and then the material is quenched to cause the carbon to be retained in the austenite – instead of the carbon being allowed to form new carbides.
Because of these two impressive characteristics of manganese steel, it is typically the go-to material for crushing applications. The combination of high impact toughness and the work hardenability of the material is what makes manganese steel such a great material for crushing. In crushing applications, most steels would permanently deform or they would shatter. Meanwhile, properly produced manganese steel will take the impact like a champ and continue to get harder and tougher as it is continually exposed to impact. Temperform produces swing hammers, ring hammers, crusher hammers, breaker blocks, and other types of manganese steel castings for crushing applications.
Wear-resistant applications for manganese steel
Another area where manganese steel tends to perform very well is in wear-resistant applications where other alloys, particularly white irons/abrasion resistant cast irons, fail. Though white irons have a much higher hardness than manganese steel in the supplied condition, they are extremely brittle, and thus do not take impact well (or at all). If the wear-resistant application involves any significant quantity of impact, manganese steel will typically work harden over time, and as it hardens it will become almost as hard as some white irons.
A fun fact about manganese steel, and something that I personally think is massively overlooked, is that with the proper process parameters in place, manganese steel can actually become a fairly high-strength and higher yet elongation material, in addition to retaining the impact toughness. Obtaining higher strength and elongation in manganese steel is all about grain refinement, and Temperform has a process in place to achieve grain refinement in manganese steel, producing better properties. Grain refinement in manganese steel can increase the tensile strength by 25-35% and increase the elongation by 50-80%. The yield strength, it is unclear to what degree that is affected, but it is clear that grain refinement does provide some benefit to yield strength in manganese steel.
Castability of manganese steel
In terms of castability, manganese steel is not a very difficult material to produce, however there are a few quirks that give it some occasionally unique challenges. One of the greatest challenges with manganese steel arises in the molding process, that being the mitigation of manganese silicate formation during pouring. Most no-bake steel foundries in the United States are molding with silica sand, and one of the issues with manganese steel is that the manganese oxides present in the metal when it is poured will react with the silica sand to form manganese silicate. The reaction between manganese oxides and silica to produce manganese silicate is an exothermic reaction that releases heat, it breaks down the bonded sand mixture by releasing the silica from the binder at a faster rate than in the normal casting process, which then can cause excessive burn-on. In addition to that, manganese silicate will form on the surface of the casting, which is a shiny type of defect that is impossible to remove without extensive amounts of excavation and welding.
In order to combat this, foundries need to find a way to prevent the silica sand from coming into contact with the manganese steel. There are a couple of methods by which this can be achieved, the first is a specialty liquid refractory coating that gets applied to the mold. This specialty refractory coating must be applied very thick, which causes problems of its own, and even if it is applied properly, it may still not fully stop the reaction between the steel and the sand, leading to undesirable quality details on the casting surface. The second method by which the reaction between manganese steel and silica sand can be stopped is by facing a silica sand mold with a different type of sand. Facing is the practice in which a different type of sand is applied to the surface of the pattern during molding, so the sand that touches the part is not silica, but of a different mixture. Common types of facing sands that are used are olivine, ceramic, and chromite, Temperform uses chromite facing sand in their process and achieves consistently good results.
Another interesting challenge of manganese steel casting production that is often overlooked is the removal of risers and gating. With manganese steel, you absolutely cannot apply heat to it in the as-cast state, the heat will cause the material to crack. Temperform tends to use arc air to remove most of their risers, so we have to either find a different way to remove the risers and gating or we have to wait until the castings return from heat treatment to cut-off the risers with arc air. In smaller manganese steel castings, Temperform can typically remove risers by using breaker cores and a sledgehammer in the as-cast state, though with larger castings, that is not as easy. In many cases, the castings actually have to be heat treated with the rigging on, that way the material can be cut with a cut-off wheel, a chop saw, or the arc air once it returns from heat treatment. Even in the heat-treated state, care must be taken to not heat up manganese steel above 500° Fahrenheit, as doing so will reduce the impact toughness by reforming some carbides. Temperform takes very large steps to avoid over-heating manganese steel when removing rigging post heat-treatment.
Heat treatment for manganese steel
This is my favorite part about making manganese steel, it is also the most unique and quite frequently the most frustrating aspect of making manganese steel. In the simplest terms, manganese steel receives a solution anneal heat treatment with a water quench. The goal of heat treatment in manganese steel is to fully dissolve the carbides that form in the as-cast condition. In order to dissolve those carbides, manganese steel is often heat-treated at temperatures in excess of 2000° Fahrenheit to ensure full carbide dissolution. After that initial heating and transformation, manganese steel is then rapidly transferred to a water quench, which allows it to cool extremely rapidly. The fast-cooling rate causes the carbon to be retained in the austenite, which stops the formation of new carbides from occurring during cooling. This retention of carbon in the austenite is what gives manganese steel its ability to work harden and gives it the incredible toughness properties that it is so well known for.
However, as my father says, “If it was that easy, everyone would do it.” Among the many unique and interesting things about manganese steel, one of the most fascinating is the thermal properties of the material, particularly at and under 650° Celsius. The thermal conductivity and the coefficient of thermal expansion for manganese steel are very different than other steels, which poses some extremely unique challenges during heat treatment. Compared to normal alloy steel, manganese steel has about a 33% lower thermal conductivity at temperatures under 600° Celsius, which means that it does not heat as evenly as other steels. Why is this a problem? It is a problem because when you heat manganese steel rapidly, or even semi-rapidly, it forms large thermal gradients in the material, which puts internal stresses on the material, and can often cause castings to crack. This is the precise reason why heat cannot be applied to manganese steel in the as-cast condition, because the rapid application of that heat will cause cracking.
As a result of these unique thermal properties, special care needs to be taken when heat treating manganese steel. The first important thing to note is that you must have very good temperature uniformity in the heat-treatment furnace throughout the entirety of the cycle. Any lack of uniformity may cause certain parts of the casting to get hotter faster than other parts, which can and will cause cracking. Another important thing to think through very carefully is the ramp rate when heat treating manganese steel, especially thick castings. Sometimes, the ramp rates on manganese steel need to be painfully slow, but often if they are not, the castings will crack, sometimes coming out of the furnace in two pieces (Speaking from experience, not something that I remember fondly, but I did learn!)
Temperform has a robust heat-treatment structure designed for manganese steel to suit every size casting that is capable of being cast at Temperform.
Look to Temperform for your manganese steel needs
When it comes to manganese steel, it is an intricate material that takes a lot of knowledge, understanding, and patience to cast. That being said, Temperform has invested heavily in building up their knowledge of manganese steel in order to optimize their process to provide customers with the best manganese steel castings possible. If you are looking for manganese steel castings under 5,000lbs that are made to the highest standard, look no further than Temperform and speak to one of our steel experts today!