Here are 8 reasons why you might want to consider stress relieving the steel before machining your parts.
High carbon grade of steel. Alloy grades over 0.40 carbon and carbon grades above 0.50 carbon can often benefit from stress relief.
Heavy draft to make size. Heavy draft can add cold working strain which can set up stresses in the part.
Small diameter parts. The percentage of cold work (strain) is higher for the same draft reduction as diameter decreases.
Long parts. Stresses tend to display and their effects increase longitudinally.
Assymetric parts– and parts with large differences in section or mass.
To increase mechanical properties. At lower stress relieving temperatures, the hardness, tensile strength, and elastic properties of most cold drawn steels increase.
To decrease mechanical properties. At higher stress relieving temperatures, hardness, tensile strength and yield strength are reduced while % elongation and 5 reduction of area are increased.
To reduce distortion off the machine. Usually stress relieving is used as a last ditch effort to reduce the distortion that presents after machining a part with some or many of the characteristics given above.
Stress relieving is a lower than the material’s critical point thermal treatment also known as strain drawing, strain tempering, strain annealling, strain relieving, or pre-aging. It is performed to modify the the magnitude and distribution of of residual forces within a cold drawn steel bar, as well as to modify the mechanical properties.
It is commonly held knowledge by most people that alloy steel is “stronger” or “better” somehow than “ordinary steel.” What makes a steel “alloy steel?” What makes alloy steel “different?”
Steel is classified as an alloy steel when the maximum content of manganese exceeds 1.65%; silicon exceeds 0.5%; copper exceeds 0.6%, or in which a definite range or minimum quantity of the following elements are specified:aluminum, boron, chromium (up to 3.99%), cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium.
These elements alter the steel’s response to heat treatment, resulting in a wide range of possible microstructures and mechanical properties.
Alloying elements are always metallic- thus sulfur, phosphorus, carbon and nitrogen are NOT alloying elements.
Alloying elements are added to the steel for the purpose of increasing resistance to corrosion or chemical attack, improve hardness, improve hardenability, or to alter strength.
While the carbon content of steel is the best predictor of its properties, alloying elements are the ingredients that give a particular composition its own particular set of properties.
Key commercial takeaway
Alloying elements typically do not alter the properties of the steel until heat treated. So if someone is purchasing alloy steel and the application does not call for a heat treatment, further inquiry into why they are paying extra for alloy steel is in order.
Some things you want to have bubbles, some you don’t.
In beermaking, yeast consumes the sugars in the wort and convert them into CO2 gas bubbles- carbonation.
In steel making the main reaction is the combination of Carbon in the melt with Oxygen to form a gas. At the high temperatures involved, this gas is very soluble in the molten bath.
If the Oxygen that is available for this chemical reaction isn’t completely removed before the steel is cast the gases will continue to be forced out of the melt during solidification, resulting in porosity in the steel.
In order to control the evolution of gas, chemicals called deoxidizers are added to the steel. These chemicals, Silicon or Aluminum, Vanadium, Columbium, Niobium scavenge the available oxygen in the molten steel, react chemically to form solid oxide particles dispersed throughout the steel, rather than bubbles of Carbon Dioxide.
The amount and type of deoxidizer added determines the type of steel. If sufficent deoxidizers are added, no gas is evolved from the solidifying steel, and the steel is said to be “killed.” The ingot drawing labelled number 1 shows a fully killed (deoxidized) steel showing only a shrinkage cavity, and no bubbles or porosity. ( This shrinkage cavity would be cropped off in normal rolling practice.)
Killed steel has more uniform chemical composition and properties than rimmed, semi-killed, or non-killed steels, and generally less segregation. The uniformity of killed steel and and its freedom from porosity makes these steels more suitable for critical components and for applications involving heat treatment.
Killed steels generally contain 0.15-.35 weight percent Silicon as a deoxidizer, and may contain some of the other elements as mentioned above. These other elements may be used as deoxidizers or as grain refiners. Steel grades with a Carbon maximum of 0.30 weight % and above, and all alloy steels are typically provided as “killed steels.” Free machining steels such as 12L14, 1215, and some 11XX series steels are not “killed” with Silicon, Aluminum, etc., due to their deleterious effects on tool life and machinability. The high amounts of Manganese in these steels form Manganese Sulfides to promote machinability, and also the Manganese scavenges excess Oxygen, preventing evolution of CO2.
Austenitic Grain Size is a material characteristic that is usually reported on test reports and certification documents for the steel materials that we machine in our shops.
Coarse Austenitic Grain Size is a result of NOT ADDING grain refining elements to a heat of steel. Because these Grain refining elements have not been added, the steel has a “Coarse Austenitic Grain Size.”
Typically this practice is applied to free machining grades such as 11XX and 12XX steels. These steels are sold primarily for their ability to be machined at high production rates.
What does Coarse Austenitic Grain Size imply for the parts that you make?
Better Machinability– Coarse Grained Steels are more machinable and provide longer tool life than Fine Grained Steels. (The elements added to make the Austenitic Grain size fine create small, finely dispersed hard abrasive particles in the steel)
Better Plastic Forming– than Fine Grained Steels
More Distortion in Heat Treat- than Fine Grained Steels
Lower Ductility at the same hardness- than Fine Grained Steels
Deeper Hardenability– than Fine Grained Steels
Coarse Austenitic Grain Size will show up on the test report as an ASTM value of 1-5. Values of 5 and higher are called Fine Grained Steels, and are the result of additions of Aluminum, Vanadium, or Niobium in North American commercial practice for most Carbon and Alloy steels.
The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods For Determining Average Grain Size.
A nice discussion can also be found HERE.
While we think that chemistry may be the controlling factor for machining performance of the steel in our machines, the contribution of austenitic grain size is also important. As long as you are ordering your free machining steels (11XX and 12XX series) to Coarse Grain Practice, Austenitic Grain Size should not be an issue in your shop.
In North America, the AISI/SAE steel grade nomenclature system is widely used.
In this system, 4 numeric digits (XXXX) describe the base grade. The first two digits tell you whether the steel is a carbon or alloy grade. If the first digit is any number other than a “1”, that steel is an alloy steel. We’ll discuss alloy steels in a later post. If the first digit is a ” 1 “, the steel is a carbon grade. 10XX is the template for the plain carbon steels. We’ll explain those last two digits at the end of our post. (Exception: if the second digit is a “3”- then its one of the alloy manganese 13XX grades- grades we don’t encounter very often these days.) If the second digit is a “1”, the steel is a resulfurized carbon steel. 11XX. Guess how many “extra” elements were added to the grade? If you guessed 1- thats right. Sulfur is the one element added to promote machinability in the 11XX grades of steel. If the second digit is a “2”, the steel is called a rephosphorized and resulfurized steel. Both sulfur and phosphorus,-2 elements- are added to make these free machining steels. 1215 and 12 L14 are the grades we mostly see today. (As many of you know, that “L” as an infix tells us that there is a lead addition in the 12L14 steel.) If the second digit is a “5” the grade is a high manganese carbon steel. Grades 1524, and 1541 come to mind as the principal 15XX grades seen by our industry.
A “B” infix tells us that the steel has been treated with boron. This makes it especially adept at being heat treated. 15B21 is used to make fasteners that are heat treated.
So, now that you know what the first 2 digits mean in a US grade designation for steel, what about the last two?
The last 2 digits in the grade are the mean or average carbon content of the steel. In weight percent. So grade 1018, is a plain carbon steel, 0.18% average carbon content. 1144 is a resulfurized 0.44% average carbon content steel for higher strength and machining.
And 1215, well- 1215 is a resulfurized, rephosphorized 0.09 max weight % carbon steel for machining. 0.09% max! Don’t you just love exceptions?
When machining carbon and alloy steels, Crater Wear is the normal tool failure mode. Overheating is an unpredictable failure mode. It can be one of two failure modes, Thermal Checking ( or Cracking- my first boss called it “Crazing” ) or Deformation. Usually, when an irate customer ran into overheating issues, the tool they sent back to me had deformed to the point that it looked like it had been made out of lava.
The lack of predictability of failure by overheating creates issues for the shop beyond the obvious. Parts produced immediately prior to failure are suspect and must be validated prior to release, to avoid sending rejectable product to customers. Overheating can thus be a “delivery problem” in your customer’s eyes.
Here are 5 tips to get out of Overheating Tool Failure Mode and back to normal predictable Crater Wear Tool Failure Mode when machining steel:
Improve lubrication coolant delivery or formulation. Sometimes adding an extra coolant line to the position will eliminate the problem. Confirming your coolant is up to spec should be done before electing to buy a new “super duper formulation.” First things first!
Use a harder grade of carbide with more Ti (Titanium)
Increase the Feed Rate (IPR) inches per rev
Reduce the Speed (SFM)
Consider Ceramic or Cermet Tooling. Note- these are not really appropriate for low carbon (less than 0.20% C) steels. Low carbon steels become gummy and stringy at speeds typically used for ceramic tools.
These tips will address your overheating problem by reducing the friction, surface adhesion, and improving removal of heat, (improved coolant, delivery); improving the tool’s ability to withstand cutting conditions, and reducing the heat inputs by decreasing speed and increasing feed.
For more great information on this subject look at this lesson from Fox Valley Technical College.