QUALITY CONTROL IN HEAT TREATMENT & TESTING

Though it is difficult to define quality, it is a commonly used word in day-to-day life. In general, quality signifies product quality, and can be defined as

·         Fitness for use

·         Conformity to specification

·         Customer’s preference

·         Degree of excellence

The variables that affect heat treatment process have already been dis­cussed in earlier Chapter. The effect of some of these factors on the quality of heat-treated product is now discussed in detail.

  PRODUCT DESIGN

Product designing for heat-treating plays an important role. A properly designed part ensures satisfactory and economical heat treatment. Following factors should be given due consideration during designing

1.    There should be, as far as possible, no sharp corners.

2.    No significant variation should be present in section sizes.

3.    No sharp corners should be seen between thick and thin sections.

4.    Use of fillets at all corners and junctions should be made a routine practice.

5.    Product design must be free from single internal or external key way.

6.    There should be no clustering of openings or holes.

In addition to the above-mentioned general factors, special precautions are undertaken while designing gears, cutters, dies, shafts and threaded sections

  HEAT TREATMENT SPECIFICATIONS

Heat treatment specifications should also be provided along with design specifications or separately. These specifications should consist of details about various parameters such as hardness, tensile strength, toughness, microstructure, case and core properties, depth of decarburization, and appear­ance of fracture and grain size

Once the design of the product has been either finalized or procured from the customer, the foremost job is to select a suitable material for the product. The material selected should be able to meet the service conditions and/or technical specifications in heat-treated condition. In general, service conditions are not well defined by the customer and in such cases choice of the material lies in the hands of manufacturer. The customer is only interested in technical specifications

The most suitable material is one, which, in addition to conforming to technical specifications, is also economical. A material is said to be econo­mical if its properties just match with the properties listed in the given specification sheet. However, the concept of an economical material is somewhat complex when heat treatment is taken into account. In such a case, the economy of material selection also includes cost of heat treatment and of post-heat treatment operation(s). Some of the post-heat treatment operations are stress relieving; scale removing, machining or grinding and straightening is quite possible that the overall cost of finished heat-treated product may be much higher for cheaper material than for costly material

After selecting the material, it should be tested. In general, though the supplier produces a test certificate indicating the characteristics of the material, it is desirable to test the material at one's end. The standard methods should preferably be adopted for testing the material. However

Mutually agreed upon methods will also serve the purpose. Similarly the sampling technique and the sample size selected should either be as per standard practice (such as the one laid by ISI) or according to mutual consent. Depending on the nature of the product and service conditions, both destructive and non-destructive tests may be performed

  DIMENSIONAL CONSIDERATIONS

Due attention should be paid towards dimensions of the part before and after heat treatment. During heat treatment, parts may undergo dimen­sional changes due to thermal fluctuations and phase transformations. Dimensional changes can lead to excessive distortion in the component. The problem can be controlled by providing allowances in dimensions. The magnitude of allowances depends largely on the nature of the material itself and on the heat treatment process variables, especially temperature and rate of heating and cooling. Allowances should be provided carefully. More allowances need excessive machining or grinding which is not economical because (i) more material is wasted, (ii) more time is needed, (iii) more tools are consumed, and (iv) men and machines are engaged for longer periods. On the other hand, low allowances may not serve the purpose, and in the worst case, heat treated component may have to be rejected

  SELECTION OF EQUIPMENT, INSTRUMENT AND AUXILIARY ACCESSORIES

Selection of equipment in heat treatment is governed by following factors

1.    Nature of the operation - batch type or continuous type

2.    Desired surface characteristics of the product

3.    Heat treatment process

Batch type furnaces can be used successfully for different heat treat­ment processes and for varying sizes of the products. For these reasons, these furnaces are very popular in heat treatment plants based on job orders. Continuous type heat treatment

furnaces are well suited for mass production units. The surface of the parts heat-treated in muffle and air chamber fur­naces generally get oxidized. Another problem related to surface is decar­burization. To avoid this, saIt bath furnaces or controlled atmosphere furnaces should be employed for parts requiring good surface characteris­tics such as bearings, gears and tools. Vacuum furnaces are considered best for protecting surface characteristics

Some specific heat treatment processes demand for specially designed furnaces and other equipment. Examples are patenting, and bell-type, rotary and conveyor furnaces

Furnace is essentially a heating chamber. It does not provide any information about what is going on within the chamber. For this purpose some instruments are needed. Use of proper instruments and instrumenta­tion method is necessary for measuring and controlling temperature of the heat treatment furnace, dew point in case of controlled atmosphere furnace and air or gas pressures

There should be proper storage arrangement for water and oil. Water tank used for quenching should be of recirculation type so that constant temperature can be obtained. Circulation of water also provides stirring action to some extent. Oil tanks for oil quenching purpose should have mechanical stirring attachment. Similarly, other essential auxiliary accesso­ries should be provided at appropriate places so that heat treatment can be performed successfully

5  HEAT TREATMENT SHOP LAYOUT

In general, heat treatment is a part of some manufacturing unit such as foundry or wire industry. Very little attention is paid to this section as far as installation of equipment is concerned. A good plant layout results in (i) smooth functioning, (ii) minimum men movement, (iii) minimum materials movement, (iv) minimum production delays, (v) maximum flow of activities of various men, machines and materials at work, (vi) effective utilization of the space, (vii) increased production, and (viii) worker's con­venience and job satisfaction

6  HEAT TREATMENT FLOW SHEET

It has been generally observed that the best product is obtained only when every worker, in addition to knowing his work well, is made familiar with the product and characteristics desired after the operation, which he carries out. A detailed flow sheet of the process should be prepared and provided to the supervisors/operators. It will help the person concerned to understand one's own responsibility. An ideal process layout should consist of details about the following:

·         Materials specification

·         Furnace to be used

·         Temperature(s) of heat treatment and holding time(s) at heat treatment temperature(s)

·         Quantity of the material to be heat treated

·         Rate of heating

·         Heat treatment atmosphere and its characteristics such as dew point, volume fraction in case of mixed gases atmosphere, purity and feed­ing speed.

·         Quenchant and its characteristics

·         Auxiliary accessories to be used

·         Sample size and sampling technique    

·         Tests to be performed on each sample

The quality of the consumables also affects the final properties of heat-treated product. The following are a few of the commonly used consum­able items:

• Neutral salts such as sodium chloride (NaCl), potassium chloride (KCI), and barium chloride (BaCl₂). These are, used in salt bath 'furnaces.
• Charcoal, charcoal-barium carbonate mixture, sodium cyanide (NaCN), potassium cyanide (KCN), natural gas and water gas. These are used for carburizing steels.
• Ammonia, ammonia-nitrogen mixture, ammonia-hydrocarbon mixture, sodium cyanide and sodium carbonate mixture. These are used for nitriding or carbo-nitriding steels.
• Water, oil, water-oil emulsion, polymers, and salt baths. These are quenchants.

• Degreasing chemicals.

• Cleaning agents

 

7  WORKING OF EQUIPMENT

Equipment should always be in good working condition. Measuring instruments too should be able to provide accurate and reproducible results. A well-established, well-planned maintenance scheme is required for smooth and reliable working of equipment and instruments. Even on economical ground, a timely maintenance is profitable. Unplanned/untimely mainten­ance should be avoided

In addition to the above-mentioned factors, human factors such as good working conditions, efficient supervision, employment of skilled workers, management policy and quality mindedness affect the quality of finished product to a great extent

8  INSPECTION IN HEAT TREATMENT

Inspection in heat treatment can be, divided into three classes, namely, pre­heat treatment inspection, post-heat treatment inspection, and inspection during heat treatment. Various characteristics to be inspected in each class are as foIIows

i) Pre-heat Treatment.lnspection

• Size
• Chemical composition
• Macro and microstructure
• Austenitic grain size
• Hardenability
• Tensile strength, percentage elongation and reduction in area 
• Non-metallic inclusions
• Surface flaws.
• All machining operations as per drawing.

(ii) Inspection during Heat Treatment

• Heat treatment temperature
• Heat treatment time (heating time, soaking time and sometimes cooling time)
• Properties of gas in case of controlled atmosphere process
• Properties of solid, liquid or gaseous materials used in case of hardening (carburizing, nitriding and carbonitriding) processes. Properties of quenchants, if in use.

(iii) Post-heat Treatment Inspection

The characteristics to be inspected are almost the same as the ones included in pre-heat treatment inspection and also for the properties for which the heat treatment is carried out.

9  Testing

Testing of material is one of the important and essential steps for judging suitability for engineering applications after heat treatment. Most of the properties of interest are mechanical properties since heat treatment essentially alters mechanical properties. This chapter is confined only to hardness testing

10  HARDNESSTEST

The concept of hardness is quite old. All materials exhibit this property. Hardness has been defined in several ways, based on the principle and the manner in which test is conducted. About 30 methods are used for measur­ing different types of hardness. There are three general types of hardness:

• Scratch hardness,
• Indentation hardness,
• Rebound or dynamic hardness

Scratch hardness can be defined as that property of the material by virtue of which it resists wear or abrasion. Mohs' scale of hardness pro­posed by Friedrich Mohs, consists of 10 standard minerals. It is most commonly used for the measurement of scratch hardness. Each one of the minerals has been assigned a hardness number.

The softest mineral in this scale is, talc whereas the hardest is diamond. Though Mohs' scale finds wide application in the field of mineralogy, it is hardly used for metals and alloys, which may have hardness values lying between two con­secutive scratch hardness according to Mohs' scale. Thus, comparative hardness values of metals and alloys, which are of great significance, can­not be estimated.

Indentation hardness is a measure of resistance offered by a material to plastic deformation. It is measured by estimating the size of indentation. Indentation hardness measurement involves pressing (forcing) an indenter of known material and of well-defined geometry into the surface of the sample/work-piece under certain conditions. The size or depth of indenta­tion so obtained is used as hardness measuring parameter. Indentation hardness test is very common and finds varied applications in the field of metallurgy. It is most widely used for metals and alloys. Three most com­mon types of indentation hardness tests are

• Brinell hardness,
• Vickers hard­ness
• Rockwell hardness tests

Rebound hardness is a measure of resistance of the materials to strike and rebound. For determining rebound hardness, an indenter is dropped on the surface of the material under specific set of conditions. The energy of the impact or the height of rebound of the indenter forms the basis of measurement of rebound or dynamic hardness. Shore sceleroscope is the most commonly used rebound hardness tester

BRINELL HARDNESS TEST

Brinell hardness tester (see Fig 9-1A) consists of penetrating metal surface by a hardened steel ball (indenter) at a predetermined load. After removal of the load, the surface area of the indentation (see Fig. 9-1B) is measured. Brinell hardness is obtained by dividing the applied load by the surface area of the indentation. Though Brinell hardness has the same unit as of pressure, it is expressed as a number without assigning any unit. Therefore, the term Brinell hardness number (BHN) is commonly used. The mathematical formula for BHN is given by

                          P

BHN =-------------------------------------

                                             (pD / 2)  (D - Ö D² – d²)

where

P = applied load (kg)

D = diameter of the indenter (mm)    

d = diameter of indentation (mm)

In general, a hardened steel ball of 10 mm diameter at a load of 3000kg is used to determine Brinell hardness number of hard metals such as steel. For harder metals,

tungsten carbide ball is used in place of hardened Steel ball.

Smaller steel balls at lower loads are used for the measurement of Brinell hardness number of soft materials. In order to have reproducibility of results in Brinell hardness test, the specific ratio of applied load to square of the diameter of indentation ball (PI D2) is maintained. This ratio takes care of error(s) arising from the use of non-standard ball diameter and load. The ratio of 30, 10, and 2 is specified for steels, non-ferrous metals, and for very soft metals, respectively.

In accurate measurement of diameter of the indentation (d), certain pro­blems are encountered. In general, the measured diameter is either greater or lesser than the actual diameter of indentation. It is mainly due to the localized deformation of the metal at the indentation. The localized defor­mation affects the indentation diameter in two ways (see Fig. 9-1C), namely, ridging or sinking in. Ridging or piling up is generally observed in cold­ worked metals in which the measured diameter is larger than the actual diameter. Contrary to ridging, sinking in usually occurs in annealed metals and alloys and in this case the measured diameter is less than the actual diameter.

In a Brinell hardness tester, specimen is loaded hydraulically. There is a simple mechanical device to vary the magnitude of load. The diameter of the indentation is measured either by a graduated low power microscope or with the help of a graduated screen coupled with vernier scale attached with the machine itself. The surface of the specimen is polished in the same manner as is done for metallographic studies. However, wheel polish is" not needed. The specimen is fixed on the test platform as per specified arrange­ment. The point or spot at which hardness is desired is focused in the micro­scope. Now the required load is applied. This load is released after 15-30 seconds. The diameter of indentation is read with the aid of microscope. Now Brinell hardness number can be calculated either by substituting the values of different parameters in equation or with the help of a standard table correlating the diameter of indentation to Brinell hardness number for a given size of indentation ball and applied load.

VICKERS HARDNESS TEST

Vickers hardness test is also referred to as Vickers diamond pyramid test. The indenter used in this test is a square base diamond pyramid (see Fig 9-2). The included angle between opposite faces of the pyramid is 136º. The loads generally employed in this test vary from I kg to 120 kg, depending on the hardness of the material under test. Therefore, at varying loads, the same indenter can be used for the measurement of hardness of a number of metals and alloys. Just as in the case of Brinnel hardness test, Vickers hard­ness is represented as a number (free from unit). The Vickers hardness number (VHN) is obtained by dividing load by surface area of indentation.

The relation between Vickers hardness number and measured parameters is given by the equation

                                       1.854 P

                      VHN =-------------------------                              

                    D²

Where P is the applied load and D is the diagonal length of indentation. Due to the accuracy associated with this test, it is generally used for research and high precision work. The test is not very common for routine tests, as it requires a high degree of surface polishing and is time consuming

 

ROCKWELL HARDNESS TEST

This test differs from both the Brinell and Vickers tests in the sense that here the depth of penetration, and not the surface area, is used as the para­meter for arriving at the hardness value. It works on the principle that the depth of penetration varies with the hardness of material. The higher the hardness, the smaller will be the depth of penetration and vice versa. In this test, the depth of penetration is not measured. Instead of that, the hardness values can be read directly on a dial attached to the tester. The readings on the dial gauge are calibrated with respect to the depth of penetration. Thus no calculation is required. The test is rather coarse. Accuracy that can be achieved by this test is not comparable to either Brinell or Vickers hardness test. However, the test is very popular in day-t-o-day industrial practice. There are two basic reasons for this: Firstly, it is a fast process and, secondly, very small indentation is made on the surface. Therefore, even finished parts can be subjected to this test.

As in the case of Brinell hardness test, several combinations of indenters and loads are used in the Rockwell hardness test in order to determine the hardness of a number of materials varying from soft to hard. A 1200 dia­mond cone, also known as Brale indenter, or 1/16" and 1/8" diameter steel balls are generally used as an indenter. Loads of 60 kg, 100 kg and 150 kg are generally used. Depending on the combination of load and indenter, various scales are incorporated in the same dial of Rockwell hardness tester (Fig 9-3). It is very important to write the symbol while denoting Rock­well hardness number. In the absence of such a designation, Rockwell hard­ness number does not have any meaning. Table below shows some Rockwell hardness scales and their corresponding load and indenter. A minor load of 10 kg is applied prior to the application of major loads, i.e. 60 kg, 100 kg and 150 kg. Application of minor load serves a number of purposes. For example, it takes care of scratches on the surface or coarse surface finish, and reduces tendency towards ridging or sinking in

Scale	Indenter	Major Load (kg)	Dial Numerals	Applications
A	Brale	60	Black	For measuring hardness of very hard material (cemented carbide)
B	1/16” dia ball	100	Red	Copper alloys, aluminium alloys and unhardened steels
C	Brale	150	Black	Hardened steels, cast irons, titanium alloys and casehardened surfaces.
D	Brale	100	Black	Razor blades and certain case hardened surfaces