By Nick Knotts, Industrial Engineer for The Lawton Standard Co.
This blog is another continuation of our series covering the different families of steels that Temperform produces. In our last entry, we talked about corrosion resistant stainless steel, which wraps up our stainless-steel topics and brings us into my absolute favorite group of steel, carbon and low alloy steel. Carbon and low alloy steel is a very broad category with a LOT of detail, so this blog will actually be split up into two separate categories, the first one covers basic carbon and low alloy steel, and then the next one will be high strength low alloy steel.
This history of Carbon Steel
The history of carbon steel dates back over 3000 years to sometime around 1800 BC, when the discovery of iron-carbon alloys was made. The modern industrial roots of carbon steel date back to the 17th century, though it really wasn’t until the mid-1800s that it became readily available as a commercial material. Carbon and low alloy steel are terms that are sometimes used interchangeably, but they technically do not mean the exact same thing. By technicality, carbon steel is a material that contains carbon in a range of 0.1%-2.1%, and then sometimes residual amounts of silicon and manganese, as well as some impurities such as phosphorus and sulfur. In true carbon steel, the only element intentionally added is carbon, otherwise, it is considered alloy steel. Low alloy steel then covers the intentional addition of any alloying elements beyond carbon, so long as the total alloy content remains less than 5% (so, the material is still at least 95% iron).
The identification method for carbon and low alloys steels varies depending on the situation. The American Iron and Steel Institute produced a very widely known classification system that is the most commonly used in the foundry industry.
Performance of Basic Carbon and Low Alloy Steel
Moving forward, we will be only discussing basic carbon and low alloy steel, we will not be discussing high strength low alloy steel.
To classify general application carbon and low alloy steel, one would call it a low to medium strength material with moderate to high ductility and low to medium toughness. All carbon and low alloy steels are magnetic. Another note about the materials in this category is that they tend to be very weldable, many of them do not even require pre or post heating to be welded. Alloys in this group also tend to be very machinable.
As for the specifics about the mechanical properties, referencing some of the applicable specifications provides a snapshot as for what these materials are capable of. The most common specification for this type of carbon and low alloy steel is ASTM A27, which covers carbon steel castings for general applications. Within A27, there are a lot of different grades of carbon steel, which are meant to be referenced by casting buyers on purchase orders or by customer engineering/quality teams in specifications. These grades are identified by two numbers with a dash in between them, the marking scheme looks as follows:
Ultimate Tensile Strength (ksi) – Yield Strength (ksi)
So, for example, if you ordered grade 65-35, you would receive a carbon steel casting that is made from material with an ultimate tensile strength of at least 65ksi and a yield strength of at least 35ksi. ASTM A27 does not have specific ranges for ultimate tensile and yield strength values, meaning that anything above the specified minimums is acceptable. ASTM A27 covers ultimate tensile strengths of up to 70ksi and yield strengths of up to 40ksi, anything above that is covered by other specifications.
Another important specification to cover here is ASTM A216, which covers carbon steel castings that are suitable for fusion welding. There are only three grades in ASTM A216, WCA, WCB, and WCC. These materials all have a range on their ultimate tensile strength, WCA is allowed to have an ultimate tensile strength of 60ksi to 85ksi. WCB and WCC are allowed to have an ultimate tensile strength of 70ksi to 95ksi. As opposed to A27, these grades also have minimum % elongation and % reduction in area values specified to quantify their ductility.
There are a lot more specifications that cover the high strength low alloy steels than the carbon and low alloy steels for basic applications, most of which we will touch on in the next blog on high strength low alloy steel.
Applications for Basic Carbon and Low Alloy Steel
Common applications for basic carbon and low alloy steel castings are nearly endless. One of the most common applications, especially with the ASTM A-216 group of alloys, are pumps, valves, and other pressure vessels that are not required to be rustproof. Beyond that, a lot of simple brackets, housing and hub type castings, and other types of materials will be made out of the basic carbon and low alloy steel castings.
A lot of the reason to use these carbon and low alloy steel castings link back to machinability. By using a casting of this type, the material is usually very easily machinable, reasonably strong and ductile. Castings can be cast to a shape closer to the finished machined part, as opposed to machining the part out of roll stock or plate stock.
Castability of Basic Carbon and Low Alloy Steel
In terms of castability, there are some significant challenges with carbon and low alloy steel that foundries must navigate day in and day out. Most of the challenges with basic carbon and low alloy steel carry over to the high strength low alloy steels, so this will apply to both materials unless otherwise stated.
In my opinion, the largest challenge with pouring carbon and low alloy steel is the prevention of oxidation and re-oxidation. If our marketing team left me unchecked, I could probably write a 100-page dissertation on oxidation and re-oxidation in carbon and low alloy steel. That being said, I know that not everyone gets as excited about steel as I do, so I am going to try and do my best to hit on the key aspects of it here.
The effects of oxidation and re-oxidation
In a steel foundry, when we speak of oxidation, what we are really referring to is the process of materials in the molten metal reacting with oxygen to form oxide inclusions or oxygen-based gases. Oxidation is heavily influenced by two factors, the first is the amount of available oxygen to cause the oxidation reaction and the second is the reactivity of the molten bath with oxygen. Each of those factors is then influenced by a whole plethora of independent and dependent variables that control to what degree both oxidation and re-oxidation occur.
Starting with the amount of oxygen available to cause an oxidation reaction, the most common source of oxygen is the atmosphere that we breathe, which is approximately 20% oxygen. Anytime that the molten metal bath is allowed to interact with the atmosphere, some sort of oxidation reaction will occur, though what type of reaction and to what degree it occurs is extremely situational. In Temperform’s process, which is an in-atmosphere induction melting process without Argon-Oxygen-Decarburization, there are a handful of significant opportunities for the molten metal to interact with the atmosphere. The first is in the furnace, which has an open top that allows the metal bath to interact with the atmosphere. The second is during the tapping process, where all of the molten metal is poured into the ladle from the furnace. The third is in the pouring process, where the molten metal is poured into the mold. Typically, the latter two items are the more significant ones, because a large portion of the metal volume comes into contact with the atmosphere due to the turbulence that occurs during the tapping and the pouring process.
The reactivity of the molten metal is heavily dependent on the chemistry of the bath and the temperature of the bath. The reason why chemistry matters so much is based upon the fact that oxides are typically very simple molecules and will almost always only form between oxygen and one other element. When we say the metal oxidizes or re-oxidizes, really what that is referring to is one of those simple oxide molecules forming. As with all molecules, oxygen has a higher affinity for certain elements over others, and if the bath of metal has a higher presence of the elements that have a high affinity for oxygen, it will be more likely to oxidize. From the perspective of temperature, the higher it is, the more reactive everything in that metal bath will become, which increases the risk of oxide formation.
To summarize what I just said in the last three paragraphs quite plainly, oxidation and re-oxidation happens very easily, and it happens often. The fact that oxidation and re-oxidation occurs is not the important part of this process though, the important part of the process is how we deal with it.
How Temperform deals with oxidation and re-oxidation
The ways we deal with oxidation and re-oxidation are by reducing the amount of metal that is allowed to react with the atmosphere, and by altering the chemistry to reduce the likelihood of oxides forming and/or try to influence which ones form. There are some keys steps we take at Temperform, the first is that with all carbon and low alloy steels, we melt under an argon blanket, which we create by dripping liquid argon on the surface of the metal bath. This liquid argon drip creates a sort of shield between the metal and the atmosphere, which reduces the chances of oxygen reaching and interacting with the molten metal bath. We also diffuse gaseous argon up through the bottom of the melt bath via a porous plug, which forces any oxide inclusions buried under the surface of the melt up to the surface.
The second important step we take at Temperform is properly simulating and gating all new carbon and low alloy steel castings. With proper gating, we can take steps to minimize the time that the metal front has to interact with the atmosphere, and also minimize the turbulence that occurs during the pouring process that forms more oxides.
Another important step that Temperform takes to deal with oxidation and re-oxidation is performing a common steel-making process called de-oxidation. De-oxidation is a process that is performed when the metal is tapped out of the furnace into the ladle. Elements that have a very high affinity for oxygen are pitched into the ladle and the turbulence mixes them into the bath, these elements then grab oxygen that is bonded up with other molecules that don’t have as high of an affinity for oxygen. Common materials used for de-oxidation are Aluminum, Calcium, Silicon, Titanium, Zirconium, and others. These new oxides that form upon de-oxidation then float to the surface of the metal bath, where they will not be poured directly into the mold due to Temperform’s usage of teapot style ladles.
None of these methods for preventing and dealing with oxidation and re-oxidation are perfect, but all of them utilized together is usually sufficient to ensure that oxidation and re-oxidation do not significantly affect the final product.
Heat Treatment
The heat treatment of basic carbon and low alloy steels is both relatively simple, yet very detailed. In all but some very rare cases, carbon and low alloy steel will receive a heat treatment cycle of some sort. For the purpose of this blog, we will not be covering the specialty heat treatment cycles that are used to produce high strength steel, we will only be covering the more general heat treatment cycles. There are three cycles that will be covered in this blog, the first is annealing, the second is normalizing, and the third is normalizing and tempering.
Before I get into each individual cycle, I wanted to bring up one important commonality between all three of them, which is the process of austenitization and homogenization. Whenever steel is heated up past a certain temperature, a large portion of the microstructural constituents in the material turn to austenite. The temperature at which that occurs is largely dependent on the chemistry of the material, though in most carbon and low alloy steels, it is somewhere near to the 1500F-1600F region. Once the material is in the austenite phase, the micro segregation that tends to occur during the casting process is largely eliminated and the microstructure becomes mostly uniform – this is homogenization. From there, once the material is in the austenite phase, the cooling rate determines what the austenite will become at room temperature, and thus determines the mechanical properties of the material.
Annealing
The anneal cycle is typically used to create a homogeneous microstructure that is still fairly soft for machining purposes. During an anneal, the steel is heated until it is turned fully to austenite, typically somewhere between 1650F-1800F, and then the steel is allowed to soak for approximately one hour per inch of the thickest section of the casting. Following the soak, the furnace is then turned off, and the castings are allowed to sit in the furnace until they cool to approximately 1000F, at which point they are removed from the furnace. The cooling rate in an anneal is much slower than in a normalize, and results in the metal being much softer than in other heat treatment cycles. After an anneal, the steel is typically low strength and has high ductility.
Normalization
A normalize has the same first step as an anneal cycle, being that the steel is heated until it is turned fully to austenite, around 1650F-1800F, and then allowed to soak for approximately one hour per inch of the thickest section of the casting. However, unlike an anneal, where the furnace is turned off and the castings remain in the furnace, in a normalize, the castings are immediately pulled out of the furnace to open air. This “pull to air” as it is often called, results in a more rapid cooling rate than a furnace cool and achieves some hardening. The steel in the normalized condition is usually medium strength with medium ductility.
Normalize and Temper
A normalize and temper is a typical normalize cycle as described above, followed by a temper cycle. A temper cycle is a re-heating of the steel after it has cooled, but the re-heating stays below the temperature at which the microstructure even begins to turn to austenite. Tempering usually occurs at a temperature around 1000F-1200F. Tempering is also a much shorter process than holding in the austenite range, because tempering for too long will cause significant softening of the material. Most temper cycles tend to last between 30 minutes and 2 hours at temperature. After the temper is completed, the casting is pulled to air. Normalized and tempered steel is typically slightly lower strength than just regular normalized steel, but does see an increase in ductility over just plain normalized steel.
Conclusion
Carbon and Low Alloy steel, even in its most basic form, is a commonly relied upon workhorse of a material that drives global industry. The beauty of carbon and low alloy steel is in its simplicity, the mechanisms of how it works are relatively simple in its more basic form, and the uses of it are virtually endless. There are challenges with its production, but modern technology and hundreds of years of experience have allowed steel makers to largely combat these issues. If you are looking for carbon and low alloy steel castings made to the highest quality, look no further than Temperform and reach out to a steel expert today!