Section 2.4: Material Testing - Stres Control and Selection Flashcards
What document should the steel be supplied with?
Steel used in shipbuilding should be supplied with a steel mill certificate that’s approved by a Classification Society.
Extensive checks and tests are conducted to ensure that the material is suitable and that it meets the minimum standards set out by the Societies.
Steel will be graded with regard to the quality and purpose for which the steel will be used.
Properties of steel
- Good metallurgical properties - to overcome brittle fracture and fatigue.
- Reasonably good mechanical properties.
- Elastic limit of the steel must be a high proportion of the ultimate tensile strength (UTS).
- Chemical composition must be suitable for flame cutting without increasing the
hardness of the material. - Good resistance to corrosion.
Low carbon steels have a tensile strength of 400 to 490 NM/m2 and a yield strength of 235
NM/m2 and elongation of 15 -20%. - Easily joined by welding with very good control of weld defects.
- Reasonable cost.
Able to be cast.
* Heat treatable. Alloying elements can be used to change the characteristics of the steel.
* Good quality with few impurities
What are the components tht make up the steel?
- Carbon (0.18 to 0.28%) increases the hardness and strength, but the higher the carbon reduces the
ductility. - Manganese (.6 - 15%) this will increase the tensile strength, ductility and notch toughness.
- Silicon ( <0.5%) increases hardness and tensile strength without reducing the ability to weld.
- Sulphur ( >0.04% or 0.05% ) improves weldability and welding stresses.
- Phosphorus ( > 0.04 or 0.05% ) reduces ductility and toughness.
Grades of steel
Grade A - white.
* Mild steel used in the majority of the ship structures of less than 20 mm thickness, such
as bulkheads, tank tops, non-strength decks and superstructures.
* Grade B - green.
* Mild steel used for strength members of 20 -25 mm thickness.
* Grade D - red.
* A ‘notch tough’ steel because this material because this resists the spreading of cracks
and has higher strength. It is used for structures that are greater than 25 mm thick.
* Grade E - yellow.
* This is ‘extra notch tough’ steel because this is a heat-treated Grade D steel used for very
thick plating in excess of 50 mm thick. It is used for sheer strakes, bilge strakes and keel
plate - all high-stress areas of the vessel.
* Arctic D
* Special grades of steel used where part of the structure is subject to extremely low
temperatures. The ultimate tensile strength is 435 - 510 NM/m2 the yield stress is 310
NM/m2. This is used primarily on icebreakers.
Types of metallurgical testing procedures.
- The Bend Test
- The Hardness Test
- The Impact Test
- The Stress and Strain Test
The 3 common methods of tasting hardness?
- Vickers
- Rockwell
- Brinell
Describe the Bend Test.
Ensures that the steel can be formed into the required shape without defects.
Describe the Hardness Test.
The hardness test measures the resistance of the material to indentation under a specific load.
There is a good correlation between hardness and ultimate tensile strength; thus, hardness tests
are very useful for checking quality control in the production of steel plates and bars
Describe an Impact Test.
To find the suitability of the material to withstand shock loads such as wave action, collision,
wharf or tug impact. Unsuitable materials could develop brittle fractures as a result of impacts
which will cause serious hull damage.
Describe the Stress and Strain Test.
2 types of stresses on the hull.
There are two types of stress imposed on the hull of a ship.
1. Static or motionless stress caused when the vessels are in port or at anchor in still water.
2. Dynamic stress when a vessel is underway, drifting or at anchor in poor weather conditions
Describe Static Stress
Shearing force where forces act on a point, such as gravity acting down and buoyancy
acting upwards.
* Bending moments are imposed on the structure when two forces acting in one direction,
which may be the effects of buoyancy acting up and causing the vessel to ‘sag’ when the
downward force is less than the buoyancy. This can result in the ‘hogging’ of the hull
when the downward forces acting over buoyancy.
- Torsional Stress can be as a result of the twisting of the structure by the distribution of
the weight of cargo or ballast.
Describe Dynamic Stress
Dynamic stress is generally referred to as racking stress, shock, and chronic stress and
metal fatigue which can be attributed to these factors.
* Racking stress occurs when two forces acting in opposite directions attempt to deform
the cross-sectional form of the hull structure.
* Shock and chronic stress will cause metal fatigue caused by the work hardening of the
structure. This situation may result in cracking along particular fault lines. It may also
result in areas where dissimilar materials (where HT and mild steel sections have been
welded).
* Stress corrosion may occur over a period of time caused by the design of the equipment.
What is the 6 degrees of freedom motion?
i) Yawing.
ii) Heaving.
iii) Surging.
iv) Swaying.
v) Pitching.
vi) Rolling.
Materials used in ship’s seawater systems.
Stainless Steel 316 (Marine Grade) (321)
- Can suffer from deep pitting in stagnant water but providing the pipe is cleaned, the pitting can be reduced.
Last: 20 years
Titanium (50%
Last: 25 years
90/10 Cupro-nickel:
* Resistant to high seawater velocities allowing for smaller wall thickness on the pipe.
* Resistant to corrosion under stagnant flow conditions.
* Resistant to attachment of marine growth.
* Easily welded and manufacture.
* Reasonable cost when compared with titanium.
70/30 Cupro-nickel
Steel-stainless 18(Chromium)/8(Nickel)
Stainless 302/304
Galvanising - Zinc coating (Protects against corrosion)
Plain Steel
Material life expected from various materials in seawater.
* Hot dipped galvanising - 6 years.
* Copper with limited flow to one metre/second.
* 90/10 cupro-nickel - 10 years.
* 70/30 cupro-nickel - 22 years.
* Titanium - 25 years.
* Stainless steel 316 - 10 -12 years
