Hardness_test 硬度试验

COURSEWORK BRIEFING SHEET

COURSE MODULE (and code):Introduction to Materials (CEN106)

ASSIGNMENT TITLE: Lab Experiment: Hardness Test

ASSIGNMENT MARKS: 7.5 %

Lecturers responsible: Dr. Ominda Nanayakkara

Date set: Group 1-6: April 23, 2013

Group 7-12: April 16, 2013

Required date of submission: Group 1-6: May 03, 2013

Group 7-12: April 26, 2013

Penalty scheme for late submission: University Standard Penalty Scheme

Aims:

?To develop experimental and report presentation skills.

?To use common laboratory equipment.

?To compare the experimental data to the theoretical calculation.

Recommended Reading:

?William D.Callister, Jr., “Fundamentals of Materials Science and Engineering” Fifth Edition, John Wiley & Sons.

?Donald R. Askeland, Pradeep P. Phule, “Essentials of Materials Science and Engineering”, Thomson Learning.

At the end of this assignment a student should be able to:

?Be aware of standard laboratory equipment and its use.

?Know how to record, interpret and evaluate results.

?Know how to present a report including experimental results.

?Practised library skills.

1.Introduction

Hardness can be defined as resistance to penetration, abrasion, scratching, or cutting. These are related in that they all require plastic flow of the material. A variety of tests for hardness are in use, depending on which of the above situations is of most interest. Material properties, such as hardness can be altered to desired levels by various heat treatment procedures. Heat treatments affect the microstructure of the metal. Detailed descriptions of the changes to the microstructure can be found in material science textbooks.

Hardness and strength are related to bonding forces on the atomic level. Therefore, it should be expected that hardness and strength are somehow related. A theoretical relationship, which needs to consider the complex mechanisms involved when a hardness indentation is made, is not practical. However, various relationships have been experimentally observed and empirically defined. Usually the relationships are found in tabular form. It is important to note that the relationships may be somewhat different for different materials. Since the methods used to increase the strength of a material (such as heat treating, alloying, or mechanical working) also increase the hardness of the material, hardness measurements can provide a quick and easy means to check if a given strength has been obtained through a particular process.

This experiment will include Rockwell and Brinell hardness tests which measure resistance to penetration. The amount of deformation that occurs when a small, hard penetrator is pressed into a material surface at some designated load is the measure of the hardness of the material.

Rockwell hardness:

The penetrators for the Rockwell hardness tester

range from steel balls to very small diamond

tips (points). The smaller points are used for

harder materials that have a greater resistance to

indentation. There are various force scales used

for various materials. The Rockwell B scale is

suitable for soft engineering metals, and the

Rockwell C scale is appropriate for hard

engineering metals. Each scale requires a

specified tip and load. In some ranges, the

specific Rockwell tests are not valid.

To perform the Rockwell tests, the penetrator is pressed against the specimen with an initial preload to properly seat the penetrator. The remaining load is applied gradually after the dial on the hardness tester has been zeroed. After the penetrator has stopped moving into the specimen, the final position of the dial pointer indicates the Rockwell hardness number that is related to the depth of penetration.

Brinell hardness:

The Brinell test experiment uses a small steel ball which is pressed into a specimen by a defined load that is maintained for a small period of time. The Brinell hardness number (BHN ) is the ratio of the load (in kg ) to the impressed area (in mm ), and is calculated by:

BHN

2

Where,

- Test load [kg ]

- Diameter of the ball [mm ] - Diameter of indentation [mm ]

Since the Brinell number is based on the area of indentation, the diameter of the indentation must be measured. This is done with a microscope with a proper scale to get the indentation in millimetres. The larger diameter indentation corresponds to a softer material and lower Brinell number.

The Brinell hardness number can be used to predict the tensile strength of the material. The relationship between the Brinell number and the tensile strength can be found in reference handbooks therefore calculations are not generally required. The relationship between Rockwell and Brinell hardness numbers are given (see the figure) and therefore either hardness number can be used to predict tensile strength.

Both tensile strength and hardness are indicators of a metal’s resistance to plastic deformation. Consequently, they are roughly proportional for a given range. As a rule of thumb, the tensile strength as a function of the BHN for most steels (cast iron, steel) is related according to the following equation.

TS MPa 3.45 BHN

2.Students groups

Refer the ‘Lab Experiments – Schedule’ sheet.

3.Apparatus

1.Cover

2.Mask

3.Reading projection screen

4.Measuring microscope

5.Fine adjustment knob

6.Indenter

7.Specimen

8.Sliding platform

9.Protective cover

10.Hand wheel

11.Buffer adjustment hand wheel

12.Light adjustment hand wheel

13.Loading hand wheel

14.Back cover

15.Loading channel

16.Power connection

17.Switch

Data record:

Rockwell:

Rockwell Hardness

No. Specimen

type Load (N)

HR (hardness number)

1. Low

carbon

steel

2. High carbon steel

Brinell:

Brinell Hardness

No. Specimen

type Ball diameter

(mm)

Load

(N)

Scale reading

mm

= 0.004, 0.002 (mm)

= (Right – Left)

Left Right

1. Low

carbon

steel

2. High carbon steel

0.004 mm ; When the 2.5 times lens is used.

0.002 mm ; When the 5 times lens is used.

4.Report

Students can share only numerical data among group members. Each student should submit their own report based on own analysis and discussions.

Main content:

4.1.Front cover (Title, Group, Name, Date)

4.2.Introduction and Objectives.

4.3.Experimental methodology.

4.4.Data analysis, graphs, calculations and results (You may include tables and figures

accordingly).

1. Determine the Brinell hardness number (BHN) using Rockwell hardness number. Find

tensile strength (rough) values for steel samples you used.

2. Determine the BHN using the theoretical equation given.

3. Compare BHN s from above two steps.

4.5.Discuss hardness values of low and high carbon steels.

4.6.Discuss experimental errors you observed.

General

4.7.In a Brinell test why is a polished specimen surface more important for harder materials? 4.8.What are the size (area and the thickness) limitations of the specimen used to measure the

hardness?

4.9.List other hardness test methods available and discuss their advantages and disadvantages

in comparison to Rockwell and Brinell tests.

4.10.If 10 hardness tests were to be conducted on the same specimen, why would large errors

result if the measurements were all made at the same (or nearly same) point?

4.11.Conclusion / Reference.

(Above steps provide you a guide, you may write your own report in an appropriate way).

5.Submission

Submit in standard format of coursework submission. Delayed submission will be subjected to standard penalty scheme.