Mechanical
Steel is primarily an alloy of:
- Iron and carbon
- Copper and zinc
- Aluminium and magnesium
- Copper and tin
Explanation:
- Steel is primarily an alloy of iron and carbon, with carbon content generally below that of cast iron.
- Other alloying elements such as chromium, nickel, molybdenum, manganese and silicon may also be added to obtain specific properties.
Which element has the greatest influence on the basic classification of plain carbon steels?
- Carbon
- Chromium
- Nickel
- Tungsten
Explanation:
- Plain carbon steels are classified mainly according to their carbon content.
- Increasing carbon content generally increases hardness and strength but reduces ductility and weldability.
Which type of steel contains a relatively low percentage of carbon?
- Mild steel
- High-carbon steel
- Tool steel only
- High-speed steel
Explanation:
- Mild steel is a low-carbon steel.
- It generally has good ductility, toughness, formability and weldability.
- It is widely used for structural components, sheets, pipes and general fabrication.
Which property is generally associated with mild steel?
- Good ductility and weldability
- Very high hardness without heat treatment
- Very poor formability
- Extremely high carbon content
Explanation:
- Mild steel has relatively low carbon content.
- Its low carbon content gives good ductility, formability and weldability.
- It is commonly used where ease of fabrication is important.
What is the general effect of increasing carbon content in plain carbon steel?
- Hardness and strength generally increase, while ductility decreases
- Hardness always decreases and ductility increases
- All properties remain unchanged
- Melting point always increases significantly
Explanation:
- As carbon content increases, the steel generally becomes harder and stronger.
- At the same time, ductility and weldability generally decrease.
- The exact properties also depend on microstructure and heat treatment.
Which steel is generally easiest to weld among plain carbon steels?
- Low-carbon steel
- High-carbon steel
- Very high-carbon tool steel
- High-alloy tool steel
Explanation:
- Low-carbon steels are generally easier to weld because their carbon content is low.
- Higher-carbon steels have a greater tendency toward hard and brittle heat-affected zones and may require controlled preheating and cooling.
Which of the following is a low-carbon steel application?
- Structural sheets and general fabrication
- High-speed cutting tools
- High-temperature turbine blades only
- Permanent magnets only
Explanation:
- Low-carbon steel is widely used for structural members, sheets, plates, automobile components and general fabrication.
- Its good formability and weldability make it suitable for such applications.
Which type of steel contains significant amounts of elements added intentionally to improve specific properties?
- Alloy steel
- Pure iron
- Unalloyed copper
- Grey cast iron only
Explanation:
- Alloy steels contain deliberately added alloying elements such as chromium, nickel, molybdenum, vanadium, tungsten or manganese.
- These elements can improve hardenability, strength, toughness, wear resistance, corrosion resistance or high-temperature properties.
Which alloying element is particularly important for improving corrosion resistance in stainless steel?
- Chromium
- Lead
- Phosphorus
- Sulfur
Explanation:
- Chromium promotes formation of a thin, protective passive oxide film on the steel surface.
- Stainless steels generally contain at least about 10.5% chromium by mass under the commonly used definition.
Which alloying element is strongly associated with improving toughness and corrosion resistance in many steels?
- Nickel
- Sulfur
- Phosphorus
- Lead
Explanation:
- Nickel is used in many alloy and stainless steels.
- It can improve toughness and corrosion resistance and can stabilize austenite in suitable stainless-steel compositions.
Which alloying element is commonly used to improve hardenability and high-temperature strength?
- Molybdenum
- Lead
- Sulfur only
- Phosphorus only
Explanation:
- Molybdenum is used in many alloy steels to improve hardenability and resistance to softening at elevated temperatures.
- It is also important in several creep-resistant and tool-steel grades.
Which element is commonly added to tool steels and high-speed steels to improve hot hardness?
- Tungsten
- Lead
- Zinc
- Magnesium
Explanation:
- Tungsten is an important alloying element in many high-speed and tool steels.
- It contributes to hot hardness, wear resistance and high-temperature performance.
Which alloying element is commonly used to improve wear resistance and hardenability in steel?
- Vanadium
- Lead
- Tin
- Zinc
Explanation:
- Vanadium forms hard carbides and can refine the grain structure.
- It is used in tool steels and other alloy steels where strength, wear resistance and hardness are required.
What is the main purpose of heat treatment of steel?
- To modify its microstructure and obtain desired mechanical properties
- To permanently change its chemical composition in every case
- To melt the entire component
- To remove all carbon from steel
Explanation:
- Heat treatment involves controlled heating and cooling of steel to obtain desired microstructures and properties.
- It can modify hardness, strength, ductility, toughness, machinability and internal stresses.
- The chemical composition normally remains essentially unchanged during ordinary heat treatment.
Which of the following is a heat-treatment process?
- Annealing
- Rolling
- Extrusion
- Forging
Explanation:
- Annealing is a heat-treatment process involving controlled heating followed by controlled cooling.
- It is commonly used to soften steel, improve ductility and relieve internal stresses.
What is the principal purpose of annealing steel?
- To soften the steel and improve ductility and machinability
- To obtain maximum hardness in every case
- To increase residual stresses
- To melt the steel
Explanation:
- Annealing generally produces a relatively soft and ductile structure.
- It can improve machinability and relieve internal stresses.
- Cooling is generally slow, often in the furnace, depending on the particular annealing treatment.
In full annealing of steel, cooling is generally carried out:
- Slowly, usually inside the furnace
- Very rapidly in water
- Instantly in oil
- Only in compressed air
Explanation:
- Full annealing normally involves slow furnace cooling.
- The slow cooling allows formation of relatively soft equilibrium or near-equilibrium microstructures.
- This results in improved ductility and machinability compared with hardened structures.
Normalizing of steel generally involves:
- Heating above the critical range followed by cooling in still air
- Heating below room temperature
- Rapid cooling in water from any temperature
- Melting followed by casting
Explanation:
- Normalizing involves heating steel into the appropriate austenitic region followed by air cooling.
- It generally produces a finer structure and higher strength than full annealing.
- It is also used to refine grain structure and reduce the effects of previous processing.
Compared with full annealing, normalizing generally produces steel with:
- Higher strength and hardness
- Lower strength in every case
- No change in microstructure
- Zero residual stress in every case
Explanation:
- Normalizing uses faster cooling than full annealing.
- This generally produces finer pearlitic structures and consequently higher strength and hardness.
- Exact properties depend on steel composition and section size.
What is the main objective of hardening steel?
- To increase hardness and strength
- To maximize ductility
- To soften the material
- To remove all alloying elements
Explanation:
- Hardening generally involves heating steel to form austenite followed by rapid cooling, or quenching.
- For suitable carbon/alloy steels, this produces martensite or a martensitic structure.
- Hardening increases hardness and strength but may reduce toughness and ductility.
Which cooling medium is commonly used for quenching hardened steel?
- Water or oil
- Molten steel
- Sand only
- Dry cement only
Explanation:
- Common quenching media include water, brine, oil and specially formulated polymer solutions.
- Water generally provides more severe cooling than many oils.
- The choice depends on steel composition, section size and required properties.
What is the major disadvantage of very rapid quenching?
- It can produce high residual stresses and distortion or cracking
- It always produces soft steel
- It eliminates martensite formation
- It always increases ductility
Explanation:
- Rapid cooling produces large temperature gradients and transformation stresses.
- These can cause distortion or cracking, particularly in high-carbon steels and complex-shaped components.
- Appropriate quenching media and process control are therefore important.
What is tempering of hardened steel?
- Controlled reheating of hardened steel below the critical temperature followed by cooling
- Heating steel above its melting point
- Cooling molten steel in a mould
- Cold rolling of steel
Explanation:
- Tempering is performed after hardening.
- The hardened steel is reheated below the lower critical temperature and then cooled in a controlled manner.
- It reduces brittleness and residual stresses and improves toughness.
What is the main purpose of tempering after hardening?
- To reduce brittleness and improve toughness
- To maximize brittleness
- To completely remove carbon
- To melt the martensite
Explanation:
- As-quenched martensite can be very hard but brittle and highly stressed.
- Tempering modifies the martensitic structure, reducing brittleness and improving toughness.
- Tempering generally reduces hardness to some extent depending on the tempering temperature and time.
Which heat-treatment sequence is commonly used to obtain a hard but tougher steel component?
- Hardening followed by tempering
- Annealing followed by melting
- Normalizing followed by casting
- Tempering followed by soldering
Explanation:
- Hardening provides high hardness and strength.
- Tempering after hardening reduces brittleness and improves toughness.
- This combination is widely used for engineering components such as shafts, gears and tools.
Which heat-treatment process is mainly used to relieve residual stresses without significantly changing the microstructure?
- Stress relieving
- Full hardening
- Carburizing
- Quenching
Explanation:
- Stress relieving involves heating the component to a suitable temperature below the critical transformation range and then controlled cooling.
- Its primary objective is to reduce residual stresses produced by welding, machining, casting or cold working.
Which heat-treatment process adds carbon to the surface of low-carbon steel?
- Carburizing
- Nitriding
- Tempering
- Normalizing
Explanation:
- Carburizing is a surface-hardening process in which carbon is introduced into the surface of a suitable low-carbon steel at elevated temperature.
- After carburizing, the component is generally hardened to obtain a hard wear-resistant surface with a tougher core.
Which heat-treatment process introduces nitrogen into the surface of steel?
- Nitriding
- Carburizing
- Annealing
- Normalizing
Explanation:
- Nitriding introduces nitrogen into the surface of suitable steels.
- It produces a hard, wear-resistant surface and can improve fatigue performance.
- Nitriding is generally performed at temperatures lower than those used for conventional carburizing.
Which process is classified as a surface-hardening treatment?
- Carburizing
- Full annealing
- Normalizing
- Stress relieving
Explanation:
- Carburizing is a case-hardening or surface-hardening process.
- It increases the carbon concentration near the surface, allowing a hard case to be produced by subsequent hardening.
Which heat-treatment process uses rapid surface heating followed by rapid self-quenching by the cooler interior?
- Flame hardening
- Full annealing
- Normalizing
- Spheroidizing
Explanation:
- In flame hardening, the surface is rapidly heated above the austenitizing temperature using a flame and then rapidly quenched.
- The surface becomes hard while the interior remains comparatively tough.
Induction hardening uses which principle for rapid surface heating?
- Electromagnetic induction
- Chemical reaction
- Mechanical friction only
- Direct flame contact only
Explanation:
- Induction hardening uses alternating electromagnetic fields to induce electrical currents in the workpiece.
- These currents generate heat mainly near the surface.
- The heated surface is then quenched to produce a hard layer.
Which process is used to obtain a spheroidal form of cementite in high-carbon steel?
- Spheroidizing
- Normalizing
- Quenching
- Flame cutting
Explanation:
- Spheroidizing produces approximately spherical cementite particles in a ferritic matrix.
- This structure gives high-carbon steels improved machinability and relatively low hardness.
- It is particularly useful before machining or forming certain high-carbon steels.
Which microstructure is normally produced by rapid quenching of suitable austenitized steel?
- Martensite
- Ferrite only
- Coarse pearlite only
- Cementite only
Explanation:
- Rapid quenching suppresses diffusional transformations and can transform austenite into martensite in suitable steels.
- Martensite is very hard and strong but can be brittle in the as-quenched condition.
- Tempering is commonly performed afterward.
Which microstructure is generally softer and more ductile than martensite?
- Ferrite
- Martensite
- Iron carbide only
- Retained austenite only
Explanation:
- Ferrite is a relatively soft and ductile phase of iron-carbon alloys.
- Martensite is much harder and stronger but generally less ductile.
- The final properties depend on the complete microstructure and carbon content.
Which constituent is a hard iron-carbon compound commonly represented as Fe3C?
- Cementite
- Ferrite
- Austenite
- Martensite
Explanation:
- Cementite is iron carbide with the chemical formula Fe3C.
- It is hard and brittle and strongly influences the hardness and strength of carbon steels.
Which phase of steel has a face-centered cubic crystal structure?
- Austenite
- Ferrite
- Cementite
- Martensite only
Explanation:
- Austenite (γ-iron) has a face-centered cubic (FCC) crystal structure.
- FCC austenite can dissolve considerably more carbon than ferrite.
- Ferrite (α-iron) has a body-centered cubic structure at ordinary temperatures.
Which phase of iron has a body-centered cubic structure at ordinary temperatures?
- Ferrite
- Austenite
- Cementite
- Liquid iron
Explanation:
- Ferrite (α-iron) has a body-centered cubic (BCC) structure under ordinary conditions.
- Its carbon solubility is very low compared with austenite.
What is the main difference between annealing and normalizing?
- Annealing generally uses slower cooling, while normalizing uses air cooling
- Annealing always uses water quenching
- Normalizing always uses oil quenching
- There is no difference
Explanation:
- Both treatments involve heating steel into an appropriate temperature range followed by cooling.
- Full annealing generally uses slow furnace cooling.
- Normalizing generally uses cooling in still air, resulting in a finer structure and usually higher strength.
Which heat-treatment process generally gives the highest hardness among the following?
- Hardening by quenching
- Full annealing
- Normalizing
- Stress relieving
Explanation:
- For suitable steels, hardening followed by rapid quenching can produce martensite and a high level of hardness.
- Annealing and stress relieving are generally associated with softer or less stressed conditions.
- Tempering is normally performed after hardening to improve toughness.
Which alloying element is particularly important in stainless steel for corrosion resistance?
- Chromium
- Carbon only
- Sulfur
- Phosphorus
Explanation:
- Chromium is the key alloying element responsible for the passive protective film characteristic of stainless steels.
- The chromium-rich oxide film greatly improves resistance to corrosion.
Which type of steel is generally used when high wear resistance and high hardness are required for cutting tools?
- Tool steel
- Low-carbon mild steel
- Pure aluminium
- Low-carbon copper
Explanation:
- Tool steels are designed for high hardness, wear resistance and, depending on grade, hot hardness and toughness.
- High-speed steels are a specialized group of tool steels used for cutting tools operating at relatively high speeds.