Term Paper of MEC201

Lovely Professional University, Jalandhar

Topic:

Write a report on property called “hardness”. What does it depend upon, how to increase or decrease it. In what application it is important. What is the disadvantage of high hardness? How is hardness measure?

Submitted By:

Name: Abhay Kumar

Section: K4901

Roll No.: RK4901B27

Program: 162:: B.Tech-MBA(ME)

Submitted to:

Mr. Ashish K.G. Saran

*Contents:

• Introduction

• Measurement of hardness

• What depend upon hardness

• Mohr scale of hardness

• Application

• Advantage of hardness

• Hardness testing & measuring method

• References

*Introduction

What is Hardness?

The Metals Handbook defines hardness as "Resistance of metal to plastic deformation,

usually by indentation. However, the term may also refer to stiffness or temper or to

resistance to scratching, abrasion, or cutting.

It is the property of a metal, which gives it the ability to resist being permanently, deformed

(bent, broken, or have its shape changed), when a load is applied. The greater the hardness of

the metal, the greater resistance it has to deformation. In mineralogy the property of matter

commonly described as the resistance of a substance to being scratched by another substance.

In metallurgy hardness is defined as the ability of a material to resist plastic deformation.

The dictionary of Metallurgy defines the indentation hardness as the resistance of a material

to indentation. This is the usual type of hardness test, in which a pointed or rounded indenter

is pressed into a surface under a substantially static load.

*Measurement of Hardness:

Hardness is not an intrinsic material property dictated by precise definitions in terms of fundamental units of mass, length and time. A hardness property value is the result of a defined measurement procedure.

Hardness of materials has probably long been assessed by resistance to scratching or cutting. An example would be material B scratches material C, but not material A. Alternatively, material A scratches material B slightly and scratches material C heavily. Relative hardness of minerals can be assessed by reference to the Mohs Scale that ranks the ability of materials to resist scratching by another material. Similar methods of relative hardness assessment are still commonly used today. An example is the file test where a file tempered to a desired hardness is rubbed on the test material surface. If the file slides without biting or marking the surface, the test material would be considered harder than the file. If the file bites or marks the surface, the test material would be considered softer than the file.

The above relative hardness tests are limited in practical use and do not provide accurate numeric data or scales particularly for modern day metals and materials. The usual method to achieve a hardness value is to measure the depth or area of an indentation left by an indenter of a specific shape, with a specific force applied for a specific time. There are three principal standard test methods for expressing the relationship between hardness and the size of the impression, these being Brinell, Vickers, and Rockwell. For practical and calibration reasons, each of these methods is divided into a range of scales, defined by a combination of applied load and indenter geometry.

*MOHS' SCALE OF HARDNESS

The Mohs' hardness scale was developed in 1822 by Frederich Mohs. This scale is a chart of relative hardness of the various minerals (1 - softest to 10 - hardest). Since hardness depends upon the crystallographic direction (ultimately on the strength of the bonds between atoms in a crystal), there can be variations in hardness depending upon the direction in which one measures this property. One of the most striking examples of this is kyanite, which has a hardness of 5.5 parallel to the 1 direction ( c-axis), while it has a hardness of 7.0 parallel to the 100 direction ( a-axis). Talc (1), the softest mineral on the Mohs scale has hardness greater than gypsum (2) in the direction that is perpendicular to the cleavage. Diamonds (10) also show a variation in hardness (the octahedral faces are harder than the cube faces). For further information see articles from the American Mineralogist on microhardness, the Knoop tester, and diamonds.

*Applications

The main use of high speed steels continues to be in the manufacture of various cutting tools: drills, taps, milling cutters, tool bits, gear cutters, saw blades, etc., although usage for punches and dies is increasing.

High speed steels also found a market in fine hand tools where their relatively good toughness at high hardness, coupled with high abrasion resistance and fine, made them suitable for low speed applications requiring a durable keen (sharp) edge, such as files, chisels, hand plane blades, and high quality kitchen and pocket knives.

*Advantage of hardness

1-advantage of hardness testing in contrast to tensile testing

*Each reveals a useful property of metal (steel, etc.). Tensile strength indicates load carrying ability while hardness testing indicates results of heat treatment (soft is ductile and hard is more brittle). Often only the surface of a metal is treated for hardness and hardness is also related to crack propagation under certain circumstances.

*Another advantage of hardness testing is that it can be performed on lighter and less expensive equipment than tensile testing. It usually involves simply indenting a specimen with a diamond indenter as opposed to the large equipment required for tensile testing.

*Use Pencil Hardness to Your Advantage

When starting the preliminary drawings for any project it is best to keep the lines light and sketchy. 2H pencil hardness is ideal for this.

This is one thing I have definitely learned over the years that has greatly impacted my design and the process of which I work. When I was in high school there was a fellow artist's work that I admired for the skill and and overall style his pieces possessed. While watching him work one day I noticed he only used two or maybe three different pencils on his pieces: a 2B pencil and an Ebony pencil. These pencils are on the softer side of the spectrum when it comes to pencil hardness.

*Hardness Testing & measure method-

1-Rockwell Hardness Test

2-Rockwell Superficial Hardness Test

3-Brinell Hardness Test

4-Vickers Hardness Test

5-Microhardness Test

6-Mohs Hardness Test

7-Scleroscope and other hardness testing methods

Hardness Conversion or Equivalents:Hardness conversion between different methods and scales cannot be made mathematically exact for a wide range of materials. Different loads, different shape of indenters, homogeneity of specimen, cold working properties and elastic properties all complicate the problem. All tables and charts should be considered as giving approximate equivalents, particularly when converting to a method or scale which is not physically possible for the particular test material and thus cannot be verified. An example would be converting HV/10 or HR-15N value on a thin coating to the HRC equivalent.

*Property of hardness

1. Tensile Strength:

To better understand tensile strength, first recall that there are intermolecular forces (known as van der Waals forces) helping to hold long polymer chains in place. These forces are at their weakest when, due to structural irregularities, themolecules cannot fit closely together, resulting in a non-regimented, amorphous structure. Some polymers, however, have their constituent molecules aligned in very regular patterns. The combination of this regularity and the intermolecular forces may be enough to “fit” the chains into a rigid, crystalline pattern.

Tensile strength largely depends on an elastomer’s ability to partially strain crystallize when stretched. With greater crystallization comes increased strength and resistance to stress. Natural rubber is an example of an elastomer with a very regular chain structure that strain crystallizes. As a result, natural rubber has high tensile strength. Of course, the temporary nature of strain crystallization allows natural rubber to regain its original shape once the stress is removed. An elastomer with inherently poor tensile strength, such asstyrene butadiene, can be improved through the addition of highly particulate reinforcing agents.

 

2. Tear Resistance.

Tear resistance can be gauged via the same ASTM D 412 apparatus used in the testing of tensile strength, modulus, and elongation. As described in ASTM D 624, different specimen types can be used to measure both tear initiation (resistance to the start of a tear and tear propagation (resistance to the spread of a tear .Either way, the sample is placed in the tester’s grips, which then exert a uniform pulling force until the point of rupture. This force may then be divided by the specimen’s thickness to arrive at the tear resistance for that particular sample. Three separate samples are typically tested and an average calculated.

.

3. Compression Set.

Compression set is the end result of a progressive stress relaxation, which is the steady decline in sealing force that results when an elastomer is compressed over a period of time. In terms of the life of a seal, stress relaxation is like dying, whereas compression set is like death. Though it is very difficult to accurately quantify stress relaxation, compression set is easy to measure. ASTM D 395 details compression set testing for rubber that will be compressed in air or liquid media. Two methods are described (“A” for constant force;

“B” for constant deflection), but the basic methodology is substantially the same. Testing generally involves use of cylindrical disk compression set test buttons (0.49" thick by 1.14" diameter). In lieu of buttons, die-cut plied (stacked) samples (0.070" thick by 1.14" diameter) may be substituted. The buttons or plied samples are placed between steel plates. In method A the plates are then forced together using either a calibrated spring or a pre-defined external force , a bolt-tightened device and steel spacers are used. Either way, compression (normally 25% of original thickness) is held for a given time (e.g. 22 hours) at a specific temperature (e.g. 100° C), these last two variables based on anticipated serviceconditions.

4. Abrasion Resistance.

Measured as a loss percentage based on original weight, abration is the resistance of a rubber compound to wearing away by contact with a moving abrasive surface. Whereas the cutting or nicking of a seal’s surface is an instantaneous event, abrasive rubbing or scraping is much more of a progressive phenomenon that develops over time. Seals in motion are most susceptible to abrasion. Hardcompounds tend to exhibit less abrasive wear than soft compounds, but use of a harder compound can also increasefriction in dynamic seals, and increased friction generates seal-degrading heat.Because of the many potential variables (including heat fluctuation and surface contamination), abrasion resistance is hard to accurately measure. Testing typically involves the uniform application of an abrasive material (such as sandpaper) to the surface of a sample. ASTM standards describe three different abraders: D 1630 relies on a National Bureau of Standards (NBS) abrader ; D 2228 uses a Pico abrader (see Figure 22); and D 3389 (also known as Taber Abrasion) employs a double-head abrader and a rotary platform. Regardless of the specific test method, the relative amount of sample material that is lost due to abrasion is a good indication of abrasion resistance.

5-Resilience.

As detailed in ASTM D 2632, resilience (also known as rebound) refers to a compound’s ability to regain its original size and shape after temporary deformation. Resilience testing typically involves the dropping of a small weight onto a test specimen (such as a compression set button The extent to which the weight bounces back is then noted as a percentage of the initial drop height. A highly resilient material (one that can rapidly regain its dimensions) might engender a 70% rebound value, but values in the range of 40 to 50% are more typical for the majority of elastomers tested. Thoughcompounding may improve an elastomer in this area, it can also detract from good resilience, which is largely an inherent property. As a general rule, resilience is most critical in dynamic seals.

*References:

1. Correlation of Yield Strength and Tensile Strength with Hardness for Steels , E.J. Pavlina and C.J. Van Tyne, Journal of Materials Engineering and Performance, Volume 17, Number 6 / December 20082. G.L. Kehl, The Principles of Metallographic Laboratory Practice, 3rd Ed., McGraw-Hill Book 3. H.M. Rockwell & S.P. Rockwell, "Hardness-Tester," US Patent 1 294 171, Feb 1919.4. S.P. Rockwell, "The Testing of Metals for Hardness, Transactions of the American Society for Steel Treating,