Definition

Hard Iron: Hard iron is iron that is difficult to demagnetize once magnetized.

Soft Iron: Soft iron is iron that is easily magnetized and demagnetized with a small change of magnetic field.

Material

Hard Iron: Hard iron is a hard magnetic material.

Soft Iron: Soft iron is a soft magnetic material.

Magnetization

Hard Iron: Magnetized hard iron cannot be easily demagnetized.

Soft Iron: Magnetized soft iron can be demagnetized.

Applications

Hard Iron: Hard iron is used as permanent magnets.

Soft Iron: Soft iron is used as electromagnets.

.soft iron

Soft iron is iron that is easily magnetized and demagnetized with a small change of magnetic field. Soft iron does not refer to the soft nature of the metal; in fact, soft iron is also a hard, metallic iron. But unlike in hard iron, the magnetic domains shifted towards the direction of a magnetic field can be shifted back to the initial state. In other words, it is reversible. But the returned magnetic domains will align in a random manner.

Figure 2: An Electromagnet

Soft iron is used in the production of electromagnets. Therefore, the field can be turned on and off. An electromagnet can be made by coiling a wire around a piece of soft iron and connecting the two ends of the wire to a battery. When the current is running through the wire, this system acts as a magnet. Then the domains of the soft iron bar align with the direction of the applied field and, the intensity of the magnetic field is increased by several magnifications.

.ndt test

.non destructive testing

Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating materials without destroying the serviceability of the part

Visual inspection inspect for high splatter, undercut, bad stop start point, surface crack.

1) Surface inspection

• Magnetic particle
• Liquid penetrant

a) Magnetic particle

1. Testing magnetic fields to locate surface discontinuities in magnetic materials.
2. When the magnetic field encounters a discontinuity, the flux lines produce a magnetic flux leakage
3. Because magnetic flux lines don’t travel well in air, magnetic particles are applied to the surface of the part will be drawn into the discontinuity,
4. Use of a Yoke to generate magnetic field

b) Liquid penetrant test

Step 1 – Precleaning of the Surface

The 3 spray cans (aerosol) are provided for this test. First one is named cleaner.  The technician sprays the cleaner to the test object and then cleans the surface with non-used rag or cloth.

Step 2- Application of Penetrant

In the second step, the technician applies penetrant spray can to the surface which is in sharp red color. The technician needs to wait for 5 to 15 minutes depends on test procedure.

Step 3- Removal of the Excess Penetrant Liquid

In the third step, the technician removes penetrant liquid from the surface by rag or cloth and uses back and forth rubbing to clean the surface. No red color should be visible after cleaning.

Step 4 – Application of Developer

In the fourth step, the technician takes the developer spray can and agitates it and then sprays to the surface. Then he waits for 10 minutes. In this time, the defect will be visible

Step 5 – Evaluation / Interpretation

In the fifth step inspector evaluates the test result 

2) Internal inspection

• Radiography
• Ultrasonic most frequently used test methods

a) Radiographic method

• Exposing a test object to penetrating radiation so that the radiation passes through the object being inspected and a recording medium(film) placed against the opposite side of that object.
• With both, the radiation passing through the test object. causing an end effect of having darker areas where more radiation has passed through the part and lighter areas where less radiation has penetrated.
• Use of x-ray or gamma ray

b) Ultrasonic method

• Ultra-high frequency sound is injected into the part being inspected and if the sound hits a material with cracks, discontinuity some of the sound will reflect to the sending unit and can be presented on a visual display.
• By knowing the speed of the sound through the part and the time required for the sound to return to the sending unit, differences shows place of crack.
• Thus, able to find likely area of discontinue by the darker area

.dezincification

Dezincification selectively removes zinc from the brass alloy. It leaves behind a porous, copper-rich structure that has little mechanical strength. An in-service valve suffering from dezincification has a white powdery substance or mineral stains on its exterior surface. 

Reason for dezincification

Zinc is a highly reactive metal in galvanic series ranking. Zinc has a very weak atomic bond relative to other metals. Zinc galvanic polential is -.76V where for Copper it is +.34V. Simply, zinc atoms are easily given up to solutions with certain aggressive characteristics. During dezincification, the more active zinc is selectively removed from the brass, leaving behind a weak deposit of the porous, more noble copper-rich metal.

Factors which cause increased rates of dezincification are Copper-zinc alloys containing more than 15% zinc are susceptible to dezincification. high temperature, high chloride content of water, and low water speed.  Adding tin, Ni and Al in the alloy can reduce dezincification. Dezincification occurs in pump casing, impellers and in valves

.graphitization

 Graphitization is a microstructural change that occurs in carbon or low-alloy steels exposed to temperatures of about 425–550°C for several thousand hours.

It causes the metal to weaken and be susceptible to cracking failures. The steel tends to break down to form iron and carbon (graphite); carbon will migrate to the material’s grain boundaries forming graphite nodules, which causes the metal to become brittle, losing strength, toughness, creep resistance, and ductility.

Creep resistance is solid material’s ability to resist “creep”

Creep: tendency of a material to slowly deform over a long period of exposure to high levels of stress.

.ship building material .ship building material  .sbm  .ship material .shipbuilding material

Ships are made up of a variety of different metals and metal alloys. Different metals will suit different part requirements best. Below are some of the most common ship parts and the metals they are composed of.

SHELL PLATING

Shell plating is typically made of rolled plain steel of grades D or E. During the steel ship hull construction, shell plating, creates a water-tight barrier on the bottom and sides of the ship. It typically consists of several curved and flat steel plates, welded together.

SUPERSTRUCTURES

The superstructures are the part of the boat that is built up above the deck. An example of this is seen on any cruise ship. The superstructures of ships are now commonly made with aluminum alloys. This allows for the ship to be lighter as a whole than if the superstructures were made of steel, and the ship’s center of gravity is lower.

WATERTIGHT DOORS

Watertight doors are doors constructed on both sides of a watertight bulkhead. These doors are typically made of cast steel and prevent water from entering the ship. It is important that steel is used because the doors need to be capable of withstanding high pressures, if they are poorly made then a ship could flood.

STERN FRAME

The stern frame supports the tailshaft of the rudder and the propeller. In old ships, the frame used to be welded to the hull. The frame is typically fashioned from steel plates with plate sides that are stiffened for added support. In order to prevent it from corroding, it is heavily coated.

RUDDER PINTLES

Rudder pintles refer to the bolt/pin that attaches the rudder to the ship. In the past, brass pins in hardwood frames were used. But upon the introduction of steel, it was found that a stainless steel pintle is stronger and cheaper than its brass counterpart. The basic functionality of the pintle is similar to that of a door hinge.

PROPELLERS

Ship propellers are usually constructed by a copper alloy, like brass, to withstand the corrosive effects of saltwater. They are specifically designed to prevent cavitation which occurs when bubbles of water vapor collide with the propeller and create small dents. Propellers generate the propulsion force of the ship by turning in a fan-like motion.

Marine propellers are manufactured from corrosion-resistant materials as they are made operational in seawater which is a corrosion accelerator. The materials utilized for making marine propellers are an alloy of stainless steel and aluminum. Other popular materials employed are alloys of nickel, bronze, and aluminum, which are 10~15 % lighter than other materials and have higher durability.

Also

Cast steel is used in parts of rudder, stem and stern

Forged steel used in anchor, chain, rudder shaft

Non-ferrous metal – Aluminium used in compass area, superstructure, boats, copper nickel alloy (pipes)

Plastic and wood used in interior equipment and boats.

.work hardening    

.cold rolling

Types of work hardening:

.steel production method

.steel prod method

Heat Treatment Process

.heat treatment process   .htp

Hardening of steel:

.hardening of steel    

.steel hardening  .wh

→ Work Hardening

Work hardening is strengthening of metal by plastic deformation.

→ Case Hardening

Where case hardening is used

• In order to have a material that is strong and hard enough for wear resistance, surface hardening (case hardening) method is used for such purposes.
• Components such as cam and cam roller are typical parts that required strong material to take the load, while hard on contact surface for wear resistance.

.cast iron

.types of cast iron

.cast iron types

There are primarily 4 different types of cast iron. Different processing techniques can be used to produce the desired type, which include:

• Grey Cast Iron
• White Cast Iron
• Ductile Cast Iron
• Malleable Cast Iron

Cast Iron is an iron-carbon alloy that typically contains greater than 2% carbon. The iron and carbon are mixed in the desired quantities and smelted together before being cast into a mold.

Grey Cast Iron

Grey Cast iron has a graphite microstructure made up of many small fractures. It is called “grey iron” because the presence of these small cracks creates the appearance of a gray color.

Grey Cast Iron is not as ductile as other cast irons, however high wear resistance, high thermal conductivity, and the excellent damping capacity of Grey Cast Iron makes it ideal for engine blocks, fly wheels, manifolds, and liner.

White Cast Iron

The difference is that white cast iron has features cementite below its surface, while gray cast iron has graphite below its surface. The graphite produces a gray color appearance while the cementite produces a white color appearance. White Cast Iron is used primarily for its wear resistant properties in pump housings, mill linings and rods, crushers and brake shoes.

White cast iron is formed when the carbon in solution is not able to form graphite on solidification. It is a combination of pearlite and cementite cast iron. White cast irons are hard and brittle; they cannot easily be machined.  It is used in drill, taps, dies, files saw blades etc. They are unique in that they are the only member of the cast iron family in which carbon is present only as a carbide.

Ductile Cast Iron

Ductile Cast Iron is produced by adding a small amount of magnesium, approximately 0.2%, which makes the graphite form spherical inclusions that give a more ductile cast iron. It can also withstand thermal cycling better than other cast iron products. Ductile Cast Iron is predominantly used for its relative ductility and can be found extensively in water and sewerage infrastructure. The thermal cycling resistance also makes it a popular choice for crankshafts, gears, heavy duty suspensions and brakes.

Malleable Cast Iron

Malleable Cast Iron is a type of cast iron that is manufactured by heat treating White Cast Iron to break down the iron carbide back into free graphite. This produces a malleable and ductile product that has good fracture toughness at low temperatures. Malleable Cast Iron is used for electrical fittings, mining equipment and machine parts.

If the force applied is within the “Yield” of the metal, it will return to its original shape.

• When steel is cooled slowly, it has longer time for the grain to grow, hence larger grain size is produced.
• When the grain size is large, the steel becomes softer and more ductile.
• When a steel is cooled rapidly, the grain size become small, and the steel become hard and brittle.
• Ship building industry, we generally use ship steel with fine grain for balance between:
  Hardness.
  Ductility
  Strength

Steel grade : Grade of steel

.steel grade

.grade of steel

.grades of steel

Based on carbon content there are various types of steel.

1. Low Carbon Steel is steel with carbon content 0.05 – 0.3%
2. Medium Carbon Steel is steel with carbon content 0.3 – 0.8%
3. High Carbon Steel is steel with carbon content 0.8-2.0%
4. Ultra-High Carbon Steel is steel with carbon content 2.0-4.0%
5. Cast Iron is steel with carbon content >4.0%

.ships material   .ship material

.Mild Steel

Mild steel or low carbon steel (0.05 to 0.3% carbon) is used as a ship structural material. It has the advantage of having a relatively good strength weight ratio. whilst the cost is low.

There are four grades of steel in common use. They are specified by the Classification Societies as Grades A. B, D E. They arebringgraded largely upon their degree of notch toughness.  

Grade A has the least resistance to brittle fracture whilst Grade E is termed •extra notch tough•.

Grade D has sufficient resistance to cracks that’s why it is used extensively for main structural material.

Which grade of steel will be used in which part of the ship depends upon the thickness of the material and which part of the ship the material going to be used and the stress of that of the ship.

For example. the bottom Shell plating of a Ship within the midship portion of the ship will have the following grade requirements.

Plate thickness              Grade

up to 20.5 mm               A

20.5 to 25.5mm            B

25.5 mm to 40 mm       D

Above 40 mm                E

The tensile strength of the different grades remains constant at between 400 MN/m2 and 490 MN/m2.

The main difference of the grades are in the chemical composition of the steel. Chemical composition of grade D and E are such that they improve the impact strength of D and E Steels. Impact resistance is measured by means of a Charpy test in which specimens may be tested at a variety of temperatures. The minimum values required by Lloyds Register are

Type Of Steel   Temperature                 Impact Resistance

B                                        0° C                     27 joules

D                                       0°                        47 joules

E                                        -40°C                  27 joules

Higher tensile steels

As oil tankers and bulk carriers increased in size the thickness of Steel required for the main longitudinal Strength  members also increased. To reduce the thickness of material higher tensile strength is used.

These steels are designated AH. BH. DH and EH and may be used to replace the normal grades for any given Structural member. Thus, a bottom Shell Plate amidships may be 30 mm in thickness of grade DH steel.

The tensile strength is increased to between 490 MN/m2 and 620 MN /m2

High Tensile Steel used where it is most effective. For example, in upper deck plating and longitudinais, and bottom shell plating and longitudinals.

To weld HTS, low hydrogen electrodes are used, with some preheating.

Arctic D steel

If any part of the ship is going to be exposed at a particularly low temperature. then the normal grades Of steel are not suitable. A special type of steel. known as Arctic D. has been developed for this purpose. It has a higher tensile strength than normal mild steel. Its most important quality it can maintain a minimum of 40J at —55° C in a Charpy impact test.

Aluminum

• Pure aluminum is soft so it is alloyed with ni,cr,mo,zn,cu etc
• Tensile strength 260 MN/m2
• Reduced weight thus reduce fuel consumption
• Increase the deadweight
• Used in accommodation and small passenger ship and boat

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