First the HSS tool is heated to about 850°C and kept at this temperature for 4 to 5 hours. This is done to dissolve all the carbides or homogenization of WC, VC and Cr4C3. After it, tool is heated to 1200°C for 1-4 minutes. The purpose of heating to high temperature is that more the substance is cooled from high temperature to lower temperature difference, the more will be the hardness. Tool is not kept at such temperature for sufficient longer time. After this, it is quenched in salt bath to 650°C and kept at this temperature for 10-20 minutes. Direct quenching to room temperature is dangerous. Then the tool is oil quenched. For increasing the life of HSS tool, surface treatment processes are also done like, liquid cyaniding, gas cyaniding and solid or dry cyaniding.
8.19 QUESTIONS
1. Why are the Time-Temperature-Transformation (TTT) diagrams constructed?
2. How do you classify the different heat treatment processes?
3. What are the objectives of annealing?
4. Explain the various methods of annealing?
6. Write short notes on :
(a) Normalizing
(b) Tempering
(c) Mar-tempering
(d) Aus-tempering
(e) Case hardening
(f) Flame hardening
7. Discuss tempering process in detail?
8. Discuss various types of surface hardening or case hardening processes?
9. Explain the following case hardening processes:
(a) Cyaniding
(b) Nitriding
(c) Induction Hardening
(d) Types of carburizing
10. Write short notes on :
(a) Sub zero treatment of steels
(b) Age Hardening.
152
CARPENTRY
9.1 INTRODUCTION
Wood obtained from tree is the chief product of forest. It has been universally acceptable as raw material for manufacturing wooden products or appliances. From the pre-historic times, wood has been utilized an important source of getting heat by firing it. It has been utilized as an mazor construction material for making shelter for the basic need of human being. As the civilization advanced, it gained tremendous importance as special material for boat- building, for piling to support docks and railroad tracks. But in modern times, with the advance of wood chemistry, the uses of wood have recognized its importance in manufacturing cheap useful products used in day today life such as paper, furniture, textiles, plastics and hundreds of chemicals and extractives. The wooden products as plywood have superseded in some products in comparison metallic and ceramic materials. Compressed wood has also replaced some metals for gears and die casts. In war-time, in Europe, wood has been used as a source of wood gas for propelling automobiles. Similarly clothing has-been made from wood cotton and wood wool. The useful work on wood is being generally carried out in a most common shop known as carpentry shop. The work performed in carpentry shops comprises of cutting, shaping and fastening wood and other materials together to produce the products of woods. Therefore, carpentry shop deals with the timber, various types of tools and the art of joinery. In wood, there are two types of cells namely radiating outward from the center of wood cross-section and running parallel to the length of wood. Trees are generally classified into exogenous and endogenous types according to manner of growth.
Exogenous types are also known as outward growing trees which produce timber for commercial use. They grow outward and the additional growth which occurs each year takes place on the outside of the trunk just underneath its bark, while the innermost timber continues to mature. Each time the growth cycle is completed the tree gains one more growth ring or annual ring. In counting these rings, the age of a tree can be determined, as each ring represents one year of growth.
Endogenous trees are also known as inward growing. They grow inwards i.e., every fresh layer of sapwood is added inside instead of outside. Cane, bamboo and coconut are examples of such endogenous trees.
Timber is a common name imparted to wood suitable for engineering, construction and building purposes. Timber is obtained from trees by cutting the main body of tree in the suitable sizes after the full growth of tree. The timber structure is consisting of annual rings,
9
CHAPTERheartwood, sapwood, pith, cambium layer, bast, medullary rays and bark. Commercial timbers are commonly classified into hardwoods and softwoods. Hardwoods comprises of oak and beech that have a broad leaf. Whereas softwoods include pine and spruce which have narrow needle like leaf.
This chapter presents the various kinds of material, tools and equipments used in carpentry shop along with their properties and uses.
9.2 HARD AND SOFT WOOD
Hard wood is generally obtained from broad leaves or deciduous trees where as the softwood from trees having needle shaped yes or conifers. The major differences between hard wood and soft wood are given as under.
Table 9.1 Difference between Hard Wood and Soft Wood
S.No. Hard Wood Soft Wood
1 It is dark in color Its color is light
2 It is heavy in weight. It is light in weight
3 Hard woods are harder and denser. Soft woods are comparatively lighter
4 It has less resin content Few softwoods are resinous.
5 It does not split quickly It gets splitted quickly
6 It is difficult to work. It is easy to work.
7 It’s annual rings are close and often Its annual rings are well spaced and quit
indistinct distinct
8 It is slow growing. It is fast growing.
9 It has good tensile and shear resistance. It has good tensile resistance but is weak
across the fibers.
10 It does not catch fire very soon It catches fire very soon.