Page 56 - Ship Construction.DJ Eyres 6Ed

Ch05-H8070.fm Page 45 Wednesday, October 18, 2006 8:02 AM
Steels 45
NORMALIZING This is carried out by heating the steel slowly to a
temperature similar to that for annealing and allowing it to cool in air.
The resulting faster cooling rate produces a harder stronger steel than
annealing, and also refines the grain size.

QUENCHING (OR HARDENING) Steel is heated to temperatures simi-
lar to that for annealing and normalizing, and then quenched in water or
--- ใช้เพื่อการศึกษาเท่านั้น---
oil. The fast cooling rate produces a very hard structure with a higher ten-
sile strength.
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
TEMPERING Quenched steels may be further heated to a temperature
somewhat between atmospheric and 680 °C, and some alloy steels are then
cooled fairly rapidly by quenching in oil or water. The object of this treat-
ment is to relieve the severe internal stresses produced by the original hard-
ening process and to make the material less brittle but retain the higher
tensile stress.

STRESS RELIEVING To relieve internal stresses the temperature of the
steel may be raised so that no structural change of the material occurs and
then it may be slowly cooled.
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Steel Sections

A range of steel sections are rolled hot from ingots. The more common
types associated with shipbuilding are shown in Figure 5.1. It is preferable
to limit the sections required for shipbuilding to those readily available,
that is the standard types; otherwise the steel mill is required to set up rolls
for a small amount of material which is not very economic.

Shipbuilding Steels

Steel for hull construction purposes is usually mild steel containing 0.15 to
0.23 per cent carbon, and a reasonably high manganese content. Both
sulphur and phosphorus in the mild steel are kept to a minimum (less than
0.05 per cent). Higher concentrations of both are detrimental to the welding
properties of the steel, and cracks can develop during the rolling process if
the sulphur content is high.
Steel for a ship classed with Lloyds Register is produced by an approved
manufacturer, and inspection and prescribed tests are carried out at the
steel mill before dispatch. All certified materials are marked with the
Society’s brand and other particulars as required by the rules.