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Classification of Steel & Tensile Testing

Classification of Steels

The Society of Automotive Engineers (SAE) has established standards for specific analysis of steels. In the 10XX series, the first digit indicates a plain carbon steel. The second digit indicates a modification in the alloys. 10XX means that it is a plain carbon steel where the second digit (zero ) indicates that there is no modification in the alloys.

The last two digits denote the carbon content in points. For example SAE 1040 is a carbon steel where 40 points represent 0,40 oh Carbon content. Alloy steels are indicated by 2XXX, 3XXX, 4XXX, etc.. The American Iron and Steel Institute (AISI) in cooperation with the Society of Automotive Engineers (SAE) revised the percentages of the alloys to be used in the making of steel, retained the numbering system, and added letter prefixes to indicate the method used in steel making. The letter prefixes are:

 

A = alloy, basic open hearth

B = carbon, acid Bessemer

C = carbon, basic open hearth

D = carbon, acid open hearth

E = electric furnace

If the prefix is omitted, the steel is assumed to be open hearth. Example: AISI C1050

indicates a plain carbon, basic-open hearth steel that has 0.50 7o Carbon content.

Another letter is the hardenability or H-value. Example: 4340H

General representation of steels:

 

 

 

 

Table 1. Classification of steels

SAE. AISI

Number

Classification

1XXX

Carbon steels

Low carbon steels: 0 to 0.25 % C

Medium carbon steels: 0.25 to 0.55 % C

High carbon steels: Above 0.55 % Carbon

2XXX

Nickel steels

5 % Nickel increases the tensile strength without reducing ductility.

8 to 12 % Nickel increases the resistance to low temperature impact

15 to 25 % Nickel (along with Al, Cu and Co) develop high magnetic properties. (Alnicometals)

25 to 35 % Nickel create resistance to corrosion at elevated temperatures

3XXX

Nickel-chromium steels

These steels are tough and ductile and exhibit high wear resistance , hardenability and high resistance to corrosion.

4XXX

Molybdenum steels

Molybdenum is a strong carbide former. It has a strong effect on hardenability and high temperature hardness. Molybdenum also increases the tensile strength oflow carbon steels

5XXX

Chromium steels

Chromium is a ferrite strengthener in low carbon steels. It increases the core toughness and the wear resistance of the case in carburized steels.

86XX

87XX

93XX

94XX

97XX

98XX

Triple Alloy steels which include Nickel (Ni), Chromium (Cr), and Molybdenum (Mo).

These steels exhibit high strength and also high strength to weight ratio, good corrosion resistance.

 

 

Table 2. The effect of alloying elements on the properties of steel

Element

Effect

Aluminum

Ferrite hardener

Graphite former

Deoxidizer

Chromium

Mild ferrite hardener

Moderate effect on hardenability

Graphite former

Resists corrosion

Resists abrasion

Cobalt

High effect on ferrite as a hardener

High red hardness

Molybdenum

Strong effect on hardenability

Strong carbide former

High red hardness

Increases abrasion resistance

Manganese

Strong ferrite hardener

Nickel

Ferrite strengthener

Increases toughness of the hypoeutectoid steel

With chromium, retains austenite

Graphite former

Copper

Austenite stabilizer

Improves resistance to corrosion

Silicon

Ferrite hardener

Increases magnetic properties in steel

Phosphorus

Ferrite hardener

Improves machinability

Increases hardenabilifv

 

Red Hardness: This property, also called hot-hardness is related to the resistance of the steel to the softening effect of heat. It is reflected to some extent in the resistance of the material to tempering.

 

Hardenability: This property determines the depth and distribution of hardness

induced by quenching.

 

Tensile Testing

 

TENSILE TEST

 

In figure 1, point A represents the proportional limit of a material.

A material loaded with a stress in the range of "OA" will behave "elastically" and always return to its original state.

The initial segment of the curve below point A represents the elastic range and is approximated by a straight line. The slope (E) of the curve in the elastic range is defined as Young' s modulus of elasticity and is a measure of material stiffness.

 

 

A material loaded in tension beyond point A exhibits permanent deformation even when the load is removed. The proportional limit is often difficult to calculate. For this reason, two practical measurements are taken:

1.Offset yield strength or 0,2 % permanent elongation.

2. Yield by extension under load or0,5 % permanent elongation.

They approximate the proportional limit.

In figure 1, point B represents the offset yield strength and is found by constructing a line" X-B" parallel to the curve in the elastic region. Line" X-B" is offset a strain amount "O-X", which is typically 0.2% of the gage length for metals.

In figure 1, point C represents the yield strength by extension under load and is found by constructing a vertical line "Y-C". Line "Y -C" is offset a strain amount" 0-Y", which is typically 0.5% of gage length.

 

In figure 1, point D represents the tensile strength or peak stress. It is the highest stress. The test specimen will continuously become longer, even with decreasing stress.

 

In figure 1, point F (strain Z) is depicted as strain and it represents the total elongation or the amount of uni-axial strain at fracture. lt includes both elastic and plastic deformation and is commonly reported as percent elongation at break (The gage length is also reported with the result.).

 

 

 

 

Reduction of area (similar to elongation at break) is a measure of ductility and is expressed in percent. Reduction of area is calculated by measuring the cross sectional area at the fracture point (Az) and the initial one (Ao).

 

 

 

With most of the steels, an increasing reduction of area will go together with an increasing notch impact (Charpy V) property.

 

2. TEST

 

The tensile test is giving a "stress-strain curve". From this curve it is possible to calculate all the necessary mechanical properties, mentioned in the chapter 1.

 

The stress-stress curve is a graphical description of the amount of deflection under load for a given material (Fig.1 ).

 

 

 

Engineering stress (S) is calculated by dividing the load (P) at any given time by the

original cross sectional area (Ao) of the specimen.

 

 

 

Engineering strain (EL) is calculated by dividing the "elongation" of the gage length of the specimen (ΔL) by the original gage length (Lo).

 

 

The shape and magnitude of the stress-strain curve depend on the type of metal being tested.

Table1 lists average properties for selected metals. Exact values may vary widely with changes in composition, heat treating and cold working.

 

Table1 Average mechanical properties

 

Material

Modulus of elasticity

Yield strength (0,2 %)

Tensile strength

Elongation

 

GPa

MPa

MPa

%

         

Construction Steel

200

248

455

 

Hot rolled steel, 0,4 % C

207

365

579

29

Hot rolled steel, 0,8 % C

207

524

841

8

Grey (flake) cast iron

103

 

172

0,5

Annealed 18-8 stainless steel

193

248

586

55

Cold rolled 18-8 stainless steel

193

1138

1310

8

Aluminum, 2024-T-4

73

331

469

19

Aluminum, 6061-T6

70

276

310

17

Annealed Titanium

97

931

1069

13

 

Summary of terminology

 

Tensile Strength:

The maximum stress applied to the specimen. Tensile strength is also known as Ultimate Strength. (the highest point on the stress-strain diagram).

 

 

 

Modulus of Elasticity

The initial slope of the curve, related directly to the strength of the atomic bonds. This modulus indicates the stiffness of the material. (Modulus Elasticity is also known as Young's Modulus)

Modulus of Elasticity = E = Change in Stress / Change in Strain

 

 

Ductility:
The total elongation of the specimen due to plastic deformation, neglecting the elastic stretching.

 

Toughness:

The total area under the curve, which indicates the energy absorbed by the specimen in the process of breaking.

 

 

 

 

3.Testing Equipment

 

The most common testing machines are universal testers, which can test material in tension, compression, bending, and hardness.

 

Their primary function is to create the stress-strain curve. Once the diagram is generated, a pencil and straight-edge or a computer algorithm can calculate yield strength, Young's modulus, tensile strength and total elongation.

 

Testing machines are either electromechanical or hydraulic. The principal difference is the method by which the load is applied.

 

Electromechanical machines are based on a variable-speed electric motor, a gear reduction system and one, two or four screws that move the crosshead up or down. This motion loads the specimen in tension or compression. Changing the speed of the motor can change crosshead speeds. A microprocessor-based closed-loop servo system can be implemented to accurately control the speed of the crosshead.

 

Hydraulic Testing Machines (figure 2) are based on either a single or dual-acting piston that moves the crosshead up or down. However, most static hydraulic testing machines have a single acting piston or ram.

 

 

In a manually operated machine, the operator adjusts the orifice of a pressure-compensated needle valve to control the rate of loading. In a closed-loop hydraulic servo system, an electrically operated servo valve for precise control replaces the needle valve.

 

 

 

In general, electromechanical machines are capable of a wider range of tests speeds and longer crosshead displacements, whereas hydraulic machines are more cost effective for generating higher forces.

 

The most common possibilities of the 2 types of equipment are summarized in table 2.

 

Table 2 Tensile testing equipment

 

Machine Type

Test Speed

Maximum Displacement

Load Capacity

 

mm/min

mm

N

       

Electromechanical

0,0025 – 1000

1000

500 – 300000

Hydraulic

0,125 – 75

1500 – 3000

300000 – 5000000

 

All equipment must be tested regularly according to standards and be certified

by an "organization" which is allowed to do this!

 

 

4.Important Factors

 

Many factors affect the shape and magnitude of the stress-strain diagram. If they are not handled properly, errors may make the test worthless. All lab manager and test technicians should be mindful of the following common sources of error.

 

4.1 The Extensometer

When testing metals, the deflection of the load frame in comparison to the deflection of the specimen may be large enough to introduce significant error. Therefore, metals tests require and extensometer, which measures the deflection of the specimen only (figure 3).

Most extensometers are attached directly to the specimen, but non-contacting systems are also available.

 

 

 

The five most important characteristics of an extensometer are the attachment mechanism, knife edges, gage length, percent travel and accuracy.

 

Extensometer slippage, due to poor adjustment of the damping mechanism and or worn knife edges can result in an indeterminate stress-strain curve.Slippage is the most common source of error in metals testing. An appropriate maintenance program should be established to ensure that the knife edges are replaced when worn and that the springs and clips create enough pressure on the specimen.

Standard extensometer gage lengths are available. The gage length needed for a given test is dictated by the size of the specimen and the test method. Care must be taken to establish the initial gage length when attaching the extensometer. Proper adjustment and operation of the mechanical stops will eliminate gage length errors.

 

The amount of extensometer travel should match the amount of specimen elongation. An extensometer with too much travel may make it difficult to accurately measure Young's modulus. An extensometer with insufficient travel will prevent certain measurements altogether.Many test methods require a certain extensometer accuracy class (see ASTM # 83). Make sure that the extensometer meets the accuracy required prior to testing.

 

4.2 The grips

Wedge-action grips are the most common grips in metals testing. As the axial load increases, the wedge acts to increase the squeezing pressure applied to the specimen. Wedge grips are manually, pneumatically or hydraulically actuated. For high-volume testing, pneumatic or hydraulic grips are recommended.

Worn or dirty grip faces can result in specimen slippage, which often renders the stress-strain diagram useless. Therefore, the grip faces should be inspected periodically. Worn inserts should be replaced, and dirty inserts cleaned with a wire brush.

Proper alignment of the grips and the specimen when clamped in the grips is important. Offsets in alignment will create bending stresses and tower tensile stress readings. lt may even cause the specimen to fracture outside the gage length. Some test machines require backlash nuts to hold the grips in place. The backlash nuts should be tightened while a specimen loaded to machine capacity is installed in the machine.

 

4.3 The Test Specimen

Most ASTM or similar test methods require a shaped specimen that concentrates the stress within the gage length. If the specimen is improperly machined, it could fracture outside the gage length, resulting in strain errors.

 

Improper reading of specimen dimensions also creates stress measurement errors. Therefore, worn micrometers or calipers should be replaced and care should be taken when recording specimen dimensions. Some computer-based test systems read the micrometer or caliper directly, thus eliminating data entry error.

 

The elongation is depending on the ratio L/D (length to diameter) of the test specimen. An increasing L/D ratio will lead to a decreasing elongation.

If this ratio is not correct, the result will not be correct.

 

4.4. Test speed

The share and magnitude of the stress-strain diagram can be affected by the test speed.

For example, some materials exhibit an appreciable increase in strength with faster test speeds. Therefore, make sure that the load rate is in accordance with the specific test method.

 

4.5 Misalignment

In addition, worn machine components can result in misalignment, creating bending stresses that tower tensile stress readings. Check the test machine's alignment and play, to ensure concentricity of the crosshead over the full travel.

 

4.6 Incorrect "zeroed out"

Finally, with the presence of microprocessor-based test systems, applied loads can inadvertently be "zeroed out", resulting in lower stress readings. To prevent this, clamp the specimen in the upper grip, then zero the load, then dose the lower grip.

 

Note:
The transverse rupture test is a strength test designed for low-ductility material, including carbides and powder metallurgy (P/M) materials.

 

This destructive test involves bending rather than pulling of the specimen. Maximum load, specimen dimensions and test time are used to calculate the stress needed to cause failure.

 

A typical transverse rupture strength is 1,5 to 2,0 times the tensile strength.

 

 

5.Standards

 

1. European standards

 

New standards

 

EN 10002 Part1 Metallic Materials; Tensile testing; Part 1: Method of testing at ambient temperature.

EN 10002 Part 5 Metallic Materials, Tensile testing; Part 1: Method of testing at high temperature.

 

Older standards

DIN 50145 Prfuung metallischer Werkstoffe Zugversuch

DIN 50125 Prufung metallischer Werkstoff; Zugproben

DIN 51221 Teil 1: Werkstofprufmaschinen; Zugprufmaschinen, Algemeine Anforderungen

DlN 51221 Teil 2 : Werkstofprufashinen; Zugprufmaschinen, Grosse Zugprumaschinen und Universaplrufmaschnien

DIN 51221 Teil 2: Werkstofprufmaschinen; Zugprufmaschinen, Kleine Zugprrfmaschinen

 

2. USA standards

 

The following is a partial list of ASTM test methods and practices for metals testing.

 

1. Test Method E 8-00b Standard Test Methods for Tension Testing of Metallic:

Materials and Test Method E 8M-00b Standard Test Methods for Tension Testing of Metallic Materials (Metric)

 

2. Test Method E111-97 Standard Test Method for Young's Modulus, Tangent Modulus and Chord Modulus

 

3. Specification A356 / A356M-98e1 Standard Specification for Steel Castings, Carbon, Low Alloy and Stainless Steel, Heavy-Walled for Steam Turbines

 

4. Practice E1012-99 Standard Practice for Verification of Specimen Alignment under Tensile loading.

 

5. Test Method A370-97a Standard Test Methods and Definitions for Test Methods of Tension Testing of Steel Products

 

6. Test Method A345-93 (1998) Standard Test Methods and Definitions for Test Methods of Tension Testing of Metallic Foil.

 

7, Practice E29-93a (1999) Standard Practice for Using significant Digits in Test Data to Determine Conformance with Specifications.

 

8. Practice E83-00 Standard Practice for Verification and Classification of Extensometer.

 

9. Test Method E 21-92 (1998) Standard Test Methods for Elevated Temperature Tension Tests of Metallic Materials

 

 

6.Conclusion

 

The tensile test is most common test.

 

The tensile strength is the highest stress encountered in the tensile test.

This mostly corresponds with the stress at fracture. But for very ductile materials, the stress fracture is lower than the tensile strength. For very brittle materials, the yield strength equals the tensile strength.

 

The ductility is measured by the elongation at rupture as well as the reduction in area.

 

It provides the producer of the metallic components and the customer (designer of the components) with most of the required mechanical properties.

 

It is important to do the test in a correct method with equipment that is properly certified.

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