What iswork hardeningin Engineering

A cut is to be taken with a (HSS) turning tool on a 0.75 inch piece of 1045 steel with a brinnel hardness of 300. Calculate the RPM setting to perform this cut.

Hill developed a more complex relationship, in 1948, using r̅ (or r-bar), the plastic anisotropy ratio. There have been refinements over the years, and this equation suffices here:

Work hardening is characterized by the n-value—related to the slope of stress-strain curve. More formable material grades have higher n-values, meaning that for the same amount of strain created by the part design, more formable grades strengthen to a greater extent.

Since the available spindle speed settings are generally not infinitely variable, the machine cannot be set precisely to the calculated RPM setting. Some judgment must be made in selecting the speed to use. Try to get to the speed which is nearest to the calculated RPM, but if you can’t, consider these conditions. Are you roughing or finishing? If you are roughing, go slower. If you are finishing, go faster. What is your depth of cut? If it is a deep cut, go to the slower RPM setting. Is the setup very rigid? Go slower for setups that lack a great deal of rigidity. Are you using coolant? You may be able to go to the faster of the two settings if you are using coolant. The greatest indicator of cutting speed is the color of the chip. When using a high-speed steel cutter, the chips should never be turning brown or blue. Straw-colored chips indicate that you are on the maximum edge of the cutting speed for your cutting conditions. When using carbide, chip colors can range from amber to blue, but never black. A dark purple color will indicate that you are on the maximum edge of your cutting conditions. Carbide cutting tools are covered in much greater detail in other sections of your learning materials.

Metal work hardeningformula

Forming forces exceeding a material’s yield strength create the plastic deformation necessary to produce an engineered stamping. As deformation continues throughout the press stroke, metal alloys strengthen from a process known as strain hardening (or work hardening), which reduces the tendency for localized thinning in highly deformed areas. This leads to the characteristic parabolic shape of a stress-strain curve between the yield and tensile strengths. Work hardening is characterized by the n-value—related to the slope of stress-strain curve. More formable material grades have higher n-values, meaning that for the same amount of strain created by the part design, more formable grades strengthen to a greater extent. Exploiting this phenomenon, it may be possible to create a lighter-weight high-strength steel from mild steel.  While this may sound too good to be true, achieving this may require a change in forming methods from draw forming to stretch forming.  Estimating the yield strength of a formed panel requires knowledge of the forming strain and the as-received (flat sheet metal) mechanical properties determined from tensile testing. Several techniques measure forming strain, such as manual circle-grid strain analysis, camera-based noncontact analysis methods or even commercial simulation programs. These techniques may show individual strains in each direction. The formed panel yield strength (σf) can be estimated as:  where we determine n (work hardening exponent) and K (strength coefficient) from the true stress-true strain data at strains up to εu, the strain at uniform elongation. εeff is the effective strain, a way to combine the biaxial effects of ε1 (major strain) and ε2 (minor strain) into a single term.   Note: The equations shown here are for steel grades. Estimating the strength of aluminum alloys requires different equations, but the same general concept applies.   For more than 50 years, researchers have been defining terms and methodologies to best describe effective strain. The simplest way is to just define effective strain as the sum of the major and minor strains: Hill developed a more complex relationship, in 1948, using r̅ (or r-bar), the plastic anisotropy ratio. There have been refinements over the years, and this equation suffices here:   This equation provides the tools needed to choose between designing the forming process for higher-strength steel (likely also requiring a higher purchase price) or obtaining the needed strength by forming a panel made from lower-strength steel.

Note that in the R.P.M. calculation, we used the diameter of the drill and not the workpiece. This was done because the cutting takes place at the diameter of the drill, not on the outside diameter of the workpiece.

Which wheel traveled farther? The larger wheel traveled farther because it has a larger circumference and has more surface area. Cutting speeds work on the same principle. If two round pieces of different sizes are turning at the same revolutions per minute (RPM), the larger piece has a greater surface speed. Surface speed is measured in surface feet per minute (SFPM). All cutting speeds work on the surface footage principle. Again, cutting speeds depend primarily on the kind of material you are cutting and the kind of cutting tool you are using. The hardness of the work material has a great deal to do with the recommended cutting speed. The harder the work material, the slower the cutting speed. The softer the work material, the faster the recommended cutting speed (Figure 2).

Another option: Use a bake-hardening steel grade with 210-MPa minimum yield strength, which will have added formability, allowing the use of tighter die radii and the ability to hold the panel tighter without splitting. These actions lead to more stretching, increasing the amount of work hardening.

Work hardeningexamples

Reaching these levels of percent-stretch with bake-hardenable steel leads to a substantially stronger panel, approaching 350 MPa before the paint-bake cycle, which may add another 40 MPa. In-vehicle panel strength after forming and baking correlates with dent resistance, leading engineers to specify bake-hardenable grades on many Class A automotive panels.

Take this a step further: Transitioning from draw beads to lock beads holds the panel even tighter, possibly allowing the use of mild steel while still achieving the same strength in the formed part. In the example highlighted in the table, stretching mild steel by just a few percentage points results in more than a 60-percent increase in yield strength as compared to the strength of the incoming flat sheet.

A turning operation is to be done on a 3.00-inch piece of 4140-alloy steel with a brinnel hardness of 200. A carbide turning tool is to be used. Calculate the RPM setting to perform this cut.

Metal work hardeningprocess

This simplified version of the RPM formula is the most common formula used in machine shops. This RPM formula can be used for other machining operations as well.

A 1-inch (HSS) drill is used on a 4-inch diameter piece of 1012 steel with a brinnel hardness of 100. Calculate the RPM setting to perform this drilling operation.

Note: The equations shown here are for steel grades. Estimating the strength of aluminum alloys requires different equations, but the same general concept applies.

where we determine n (work hardening exponent) and K (strength coefficient) from the true stress-true strain data at strains up to εu, the strain at uniform elongation. εeff is the effective strain, a way to combine the biaxial effects of ε1 (major strain) and ε2 (minor strain) into a single term.

Let's put this formula to work in calculating the RPM for the machining example below. Use the recommended cutting speed charts in Table 4.

Work hardeningprocess

The lathe R.P.M. must be set so that the single point cutting tool will be operating at the correct cutting speed. To set the proper speed we need to calculate the proper revolution per minute or RPM setting. We stated earlier that cutting speed or surface speed would change with the size of the part. So to keep the surface speed the same for each size part we must use a formula that includes the diameter of the part to calculate the proper RPM to maintain the proper surface footage.

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Estimating the yield strength of a formed panel requires knowledge of the forming strain and the as-received (flat sheet metal) mechanical properties determined from tensile testing. Several techniques measure forming strain, such as manual circle-grid strain analysis, camera-based noncontact analysis methods or even commercial simulation programs. These techniques may show individual strains in each direction.

Exploiting this phenomenon, it may be possible to create a lighter-weight high-strength steel from mild steel.  While this may sound too good to be true, achieving this may require a change in forming methods from draw forming to stretch forming.

The hardness of the cutting tool material has a great deal to do with the recommended cutting speed. The harder the cutting tool material, the faster the cutting speed (figure 3). The softer the cutting tool material, the slower the recommended cutting speed.

Work hardeningdefinition occupational therapy

Metal work hardeningexamples

This equation provides the tools needed to choose between designing the forming process for higher-strength steel (likely also requiring a higher purchase price) or obtaining the needed strength by forming a panel made from lower-strength steel.

There are rules and principles of cutting speeds and R.P.M. calculations that apply to all metal cutting operations. The operating speed for all metal cutting operations is based on the cutting tool material and the hardness of the material to be cut. In this unit we will concentrate on cutting speeds for single point tooling.

For example, consider the task of creating a Class A panel that can withstand 260 MPa before permanent plastic deformation. One approach: Purchase a bake-hardening steel grade with 260-MPa minimum yield strength. However, this grade of steel has relatively low formability, which may limit design flexibility.

Strainhardening

For more than 50 years, researchers have been defining terms and methodologies to best describe effective strain. The simplest way is to just define effective strain as the sum of the major and minor strains:

Forming forces exceeding a material’s yield strength create the plastic deformation necessary to produce an engineered stamping. As deformation continues throughout the press stroke, metal alloys strengthen from a process known as strain hardening (or work hardening), which reduces the tendency for localized thinning in highly deformed areas. This leads to the characteristic parabolic shape of a stress-strain curve between the yield and tensile strengths.

Putting more strain into the panel results in another major benefit: Major strain, minor strain and thickness strain are related. In the areas of the panel experiencing a 3-percent by 2-percent stretch, the thickness is nearly 5 percent lower than the starting material. A 5-percent weight reduction and 60-percent strength increase using mild steel sounds like a great way to stretch your steel dollars! MF

The lathe must be set so that the part will be operating at the proper surface speed. Spindle speed settings on the lathe are done in RPMs. To calculate the proper RPM for the tool and the workpiece, we must use the following formula:

A cut is to be made with a high-speed steel (HSS) tool on a 2-inch diameter piece of 1018 steel with a brinnel hardness of 200. Calculate the RPM setting to perform this cut.

The depth of the cut and the feed rate will also affect the cutting speed, but not to as great an extent as the work hardness. These three factors, cutting speed, feed rate and depth of cut, are known as cutting conditions. Cutting conditions are determined by the machinability rating. Machinability is the comparing of materials on their ability to be machined. From machinability ratings we can derive recommended cutting speeds. Recommended cutting speeds are given in charts. These charts can be found in your Machinery’s Handbook, a textbook or in a chart given to you by your tool salesperson. In Table 4 you will find a typical recommended cutting speed chart.