Precision screwdrivers are used in the disassembly of modern smart phones - Leaving Cert Engineering - Question 3 - 2015

The heat treatment procedure for a precision carbon steel screwdriver typically involves several steps:

1. Heating: The screwdriver is heated to a temperature range of approximately 820-850°C, which transforms its microstructure into austenite.

2. Quenching: The heated screwdriver is rapidly cooled, usually in water or oil, to create a hardened structure, transforming the austenite into martensite.

3. Tempering: To reduce brittleness, the screwdriver is reheated to a lower temperature (around 220-300°C) and held at this temperature for a specific duration before cooling again. This process allows some of the internal stresses to relieve and improves toughness, making the screwdriver more resilient.

A - Liquid B - Austenite C - Ferrite and Pearlite D - Pearlite and Cementite

Point X:

• Phase change: At point X (eutectoid point), the solid austenite transforms into a solid mixture of pearlite (which is layers of ferrite and cementite).
• Composition: The composition at this point consists of 0.83% carbon, which is the critical composition for the eutectoid transformation.
• Temperature: The temperature at point X is 723°C.

Point Y:

• Phase change: At point Y, the liquid phase transforms into solid austenite during cooling.
• Composition: The composition corresponds to 4.3% carbon.
• Temperature: The temperature at point Y is 1147°C.

Optical pyrometers measure temperature based on the intensity of light emitted from an object, adjusting for variable distances and the light source. In contrast, thermocouple pyrometers utilize a junction of two different metals to produce a voltage related to temperature. This voltage is converted to a temperature reading, making optical pyrometers better suited for high temperatures and fast readings, while thermocouples are versatile in various applications.

Induction hardening involves inducing heat in the surface of a metal component using high-frequency currents, transforming its microstructure rapidly. This process is localized and allows for control over the depth of hardening. On the other hand, flame hardening uses an oxy-acetylene flame to heat the surface of a component before quenching it in water or oil. The differences lie in the methods of heating and the resulting hardness profile.

Ferrite is a solid solution of carbon in iron, containing very low carbon content (up to 0.02%). It has a body-centered cubic structure and is quite ductile. Pearlite, however, consists of alternating layers of ferrite and cementite, with a typical composition of 0.83% carbon. It provides a balance between strength and ductility, making it an essential microstructure in steel.

Soaking is the process of holding a metal part at a specific temperature for a sufficient duration to allow for uniform temperature distribution and transformation of the microstructure. This ensures the material has the desired properties. Water cooling, on the other hand, refers to rapid cooling in water after heating, which increases hardness but may introduce residual stresses, increasing the risk of cracking or warping of the treated material.