MDME: MANUFACTURING, DESIGN, MECHANICAL ENGINEERING 

HEAT TREATMENT

Steel can only be hardened by heat treated if it has enough carbon (or other alloys). Hardness comes form quenching, then toughness is improved by tempering.

Heat Treatment of Steel

In this lab several types of steel are heated treated and then tested. Silver steel, tool steel 0-1 and automotive coil spring steel are to be heat treated and hardness is tested for confirmation of properties.
  • Silver Steel: Measure hardness > heat treat > measure hardness again. Compare to charts. Temper > measure hardness. Compare again
  • Tool Steel O-1: Measure hardness of soft and hardnened samples >  Compare to charts.
  • Spring steel: Measure hardness of spring steel > Compare to charts.  Check yield stress and compared spring rate determination. (Find a Coil Spring calculator to determine stress in a coil spring mathing the given dimensions - bottom of page)  Note: since the paint is quite thick, it may require removal before hardness testing.

Heat Treatment Laboratory Procedure

For each specimen, repeat the following;
  1. Identify the material and obtain the appropriate heat-treatment schedule.
  2. Set oven to recommended temperature and heat specimen for duration specified
  3. Quench in water or oil as required
  4. Measure hardness on Rockwell Tester


Laboratory Report

The report should follow the Laboratory Report guide. Include error analysis.
Material Data The following steels are being tested;

Silver Steel (BS1407). This is an old steel definition, available in rod form with ground finish.

Analyis Range (%) Typical Analysis (%)
C .95/1.25 1.13
Si .40 max. .22
Mn .25 / .45 .37
P .045 max .014
S .045 max. .018
Cr. .35 / .45 .43

  • Hardening: Heat slowly to 760 - 800C using the upper end of the temperature range for lower carbon contents and lower end of temperature range for higher carbon contents. Austenitize until the temperature is uniform. Quench into well agitated water. The Approximate quench hardness is 65 to 68 Rc.
  • Tempering: Temper immediately after hardening preferably before the tool reaches room temperature. Temper for a minimum of 1 hour at temperatures between 180 - 350C, dependent upon the final hardness required.



Tool Steel 0-1.  
From the measurements of the part supplied, determine the probable heat treatment that has been applied.
Some relevant standards: ASTM-A-681, SAE J-437
http://www.threeplanes.net/toolsteel.html

Chemical Composition of O-1 Tool Steel      
Iron     97.1%     
Carbon     0.90%    
Chromium     0.50%    
Manganese     1.00%     
Tungsten     0.50%  
 
Physical Properties of O-1 Tool Steel
Hardening  (C)     788 - 816
Tempering  (C)     177 - 228
Hardness Range (Rc)    58 - 64

Alloying Elements - Effect on Properties
  • Carbon: Raising carbon content increases hardness slightly and wear resistance considerably. Increased hardenablility - increased yield strength.
  • Manganese: Small amounts of of Manganense reduce brittleness and improve forgeability. Larger amounts of manganese improve hardenability, permit oil quenching, and reduce quenching deformation.
  • Silicon: Improves strength, toughness, and shock resistance.
  • Tungsten: Improves "hot hardness" - used in high-speed tool steel.
  • Vanadium: Refines carbide structure and improves forgeability, also improving hardness and wear resistance.
  • Molybdenum: Improves deep hardening, toughness, and in larger amounts, "hot hardness". Used in high speed tool steel because it's cheaper than tungsten.
  • Chromium: Improves hardenability, wear resistance and toughness.
  • Nickel: Improves toughness and wear resistance to a lesser degree.

AISI-SAE tool steel grades
Defining property AISI-SAE grade Significant characteristics
Water-hardening W
Cold-working O Oil-hardening
A Air-hardening; medium alloy
D High carbon; high chromium
Shock resisting S
High speed T Tungsten base
M Molybdenum base
Hot-working H H1-H19: chromium base
H20-H39: tungsten base
H40-H59: molybdenum base
Plastic mold P
Special purpose L Low alloy
F Carbon tungsten
Table from Wikipedia


Guide to Hardening and Tempering O-1 Tool Steel

Start with annealed steel. At this stage the steel is soft enough to work with a file. Do all of your shaping now. If you're making an edge tool, however, don't grind a sharp edge yet - stop just short of sharp, leaving it blunt.

Heat the steel to the critical temperature. How do you know when you reach the critical temperature? Austenite, the iron/carbon crystal structure that forms above the critical temperature, is non-magnetic. I keep an old magnet held in a pair of vice-grips handy when hardening. When the steel is hard enough, the magnet won't stick. At this point, the steel is cherry red.

 The image actually shows dark yellow approaching bright yellow; the cherry red can be seen further down the blade.
Now, remove the steel from the heat and immediately quench in oil. Any kind of oil will suffice; I've quenched with used motor oil, but now prefer cheap vegetable oil in a metal 5 gallon pail. (I'd rather be thinking of french fries than an oil-burning motor.) When you plunge the red-hot steel into the oil, do it vertically - if you plunge it in at an angle, it will warp. Agitate it carefully in the oil, in an up-and-down motion; a stirring motion may also cause warping. 

Although it looks like I'm going in at an angle in the picture, the tongs are gripping the tool at the same angle, and the actually motion of the tool, and my arm, is vertical.
 
It's important to keep it moving to replenish the oil on the surface of the steel; otherwise a vapor layer will form resulting in a slower than desirable quench. If the quench is too slow, the tool won't be hard enough. Keep the steel in the oil until the oil stops bubbling.

As soon as the steel is cool enough to handle, wipe it off and test its hardness. If you've done right so far, a file won't bite - it'll just skate off the edge of the tool. If it's plenty hard enough, it's time to temper; it's important to temper as soon as possible after the quench. You can just put the tool in the oven if you trust its temperature setting (maybe a decent thermometer would be a good investment), or you can temper the way smiths do - by heating the tool until it reaches the right color. In order to see oxidation colors, you'll have to shine up the tool on some coarse emory paper. We're not talking mirror finish here - just enough to expose the bare metal (maybe up to about 220 grit). Now, using an appropriate heat source, carefully heat the tool from the non-business edge. The idea is to soften toward the cutting edge, so the cutting edge will be harder than the other end of the tool. For example, a knife would be harder at its edge than along its back - the back would be tempered more to give it flexibility. As the tool heats up, the first color you should see is a faint straw color. Keep heating and allow this color to spread toward the cutting edge. Just as it reaches the cutting edge, plunge the tool into some water to prevent it from tempering too much. You're done if the tool is a plane iron or chisel - all you have to do now is flatten the back and sharpen it. For a tool that needs more toughness (less brittleness), like maybe a cold chisel, you should temper a little higher. For real flexibility, like a spring, go all the way to blue.

Hardness vs Tempering Temperature

Tempering Temperature
F
Tempering Temperature
C
Approximate Hardness
Rc

300

149

63/65

350

177

62/64

400

204

61/63

450

232

60/62

500

260

58/60

600

316

55/57

700

371

51/53

800

427

48/40

900

482

44/47

1000

538

40/44


Spring Steel.  


From measurements of the piece of coil spring, determine the probable heat treatment that has been applied.

In the UK, automotive suspension coil springs are made from silicon/manganese spring steel (BS 970 251A58). [0.55 - 0.6% C, 1.8 - 2.1% Si and 0.8 - 1.0 Mn]. Typical processing gives a nominal hardness of 49.5 Rc, equivalent to a UTS of 1600 to 1700 MPa. These springs are designed for a maximum stress under full bump conditions (i.e. complete compression) of 1100 MPa.  The Japanese use a low alloy steel. [0.45 % C, 1.5 % Ni] steel with a UTS of about 1800 MPa. (From http://www.landmark-autoparts.com/coilspring/tech01.htm)

Spring manufacure: See http://www.pedders.com.au/products/56/138. The flash video is located at http://www.pedders.com.au/PS_images/assets//Coilplant.swf

This spring is probably of Japanese origin. The spring properties are; OD=173mm, wire diameter 15.3mm, coil-to-coil spacing 82mm, no or turns 6. We can use spring formulas to estimate the yield point of the steel, assuming the spring was set by full compression during manufacture.
Using the following calculator: http://www.efunda.com/DesignStandards/springs/calc_comp_designer.cfm#calc (limited trial)

A list of spring steels from http://www.acewirespring.com/technical.html (offline)
See properties_of_common_spring_materials.pdf (online)


Relevant pages in MDME
  • Mechanical Properties practice test: 10101cp
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