UPSC ESE Mechanical Engineering Manufacturing Science Syllabus

Explain the casting defects 'hot tear' (hot crack).

A hot tear is a crack that develops at high temperature when contraction of the solidifying metal is restrained by the mould or core, generating tensile stresses the weak hot metal cannot withstand.

What are 'scab', 'swell', and 'drop' defects in sand casting?

Scab: rough metal projection from sand erosion/expansion. Swell: enlargement of the cavity due to insufficient mould strength (low ramming). Drop: sand falling from the cope into the cavity, causing irregularities.

What is rolling, and what is the basic mechanism of metal flow in flat rolling?

Rolling is a bulk deformation process where metal is passed between two rotating rolls to reduce thickness. The rolls grip the workpiece by friction and squeeze it, reducing thickness while increasing length (width roughly constant).

Define the angle of bite (contact angle) in rolling and state the condition for the workpiece to be drawn in.

The angle of bite $\alpha$ is the angle subtended by the arc of contact at the roll centre. For the strip to be self-drawn in: $\tan\alpha \leq \mu$, i.e. the coefficient of friction must be at least $\tan\alpha$.

Write the expression for the maximum draft possible in flat rolling.

$$\Delta h_{max} = h_{0} - h_{f} = \mu^{2} R$$ where $\mu$ is the coefficient of friction and $R$ is the roll radius.

What is the neutral (no-slip) point in rolling?

The neutral point is the location along the arc of contact where the surface velocity of the roll equals the velocity of the strip. Before it the strip moves slower than the roll; after it the strip moves faster.

Give the expression for the length of the projected arc of contact in rolling for a draft $\Delta h$.

$$L = \sqrt{R\,\Delta h}$$ where $R$ is the roll radius and $\Delta h = h_{0}-h_{f}$ is the draft (assuming $\Delta h \ll R$).

Distinguish between hot rolling and cold rolling.

Hot rolling is done above the recrystallization temperature: large reductions, no strain hardening, poorer surface/tolerance. Cold rolling is below recrystallization: better surface finish and tolerance, increased strength via strain hardening, but higher forces.

Name common roll-mill configurations used in rolling.

Two-high, three-high, four-high (with backup rolls), cluster (Sendzimir), and tandem rolling mills.

What is forging, and how is it classified based on temperature?

Forging is a bulk deformation process that shapes metal by compressive forces (hammer/press). Classified as hot forging (above recrystallization temperature) and cold forging (below it).

Differentiate between open-die forging and closed-die (impression-die) forging.

Open-die forging deforms the workpiece between flat or simple dies that do not enclose it, giving simple shapes. Closed-die forging uses shaped dies that enclose the workpiece to produce complex near-net shapes with flash.

What is 'flash' in closed-die forging and what is its purpose?

Flash is the excess metal squeezed out into the gap between die halves. It builds up back-pressure that forces metal to fill the die cavity completely and acts as a cushion; it is later trimmed off.

What is the phenomenon of barreling in upset forging and why does it occur?

Barreling is the bulging of the sides of a cylindrical workpiece during upsetting, caused by friction at the die-workpiece interfaces restraining the ends while the middle expands freely.

Define extrusion and distinguish between direct and indirect extrusion.

Extrusion forces metal through a die to produce a constant cross-section. In direct (forward) extrusion the ram and metal flow in the same direction; in indirect (backward) extrusion the die moves toward the billet (or metal flows opposite to ram), requiring lower force as there is no billet-container friction.

Define the extrusion ratio and write its formula.

The extrusion ratio is the ratio of cross-sectional area before to after extrusion: $$R = \frac{A_{0}}{A_{f}}$$ where $A_0$ is billet area and $A_f$ is extruded product area.

Write the expression for ideal (homogeneous) true strain in extrusion in terms of the extrusion ratio.

$$\varepsilon = \ln\left(\frac{A_{0}}{A_{f}}\right) = \ln R$$ where $R$ is the extrusion ratio.

Why does direct extrusion require greater force than indirect extrusion?

In direct extrusion the billet slides along the container wall, so friction between the billet and container adds to the deformation force; in indirect extrusion there is no relative motion between billet and container, eliminating that friction.

What is hydrostatic extrusion and what is its main advantage?

In hydrostatic extrusion the billet is surrounded by a pressurized fluid that forces it through the die, with no billet-container contact. Advantages: low friction, uniform deformation, and the ability to extrude brittle materials.

Classify welding processes into the two broad categories with examples.

Fusion welding (melting, with/without filler): arc welding, gas welding, resistance welding, thermit, laser/electron beam. Solid-state welding (no melting): friction welding, forge welding, diffusion welding, ultrasonic, explosive welding.

What is the basic principle of arc welding?

An electric arc is struck between an electrode and the workpiece; the intense heat (around $5000$–$6000\,^{\circ}\mathrm{C}$) melts the base metal and filler to form a fused joint upon solidification.

In arc welding, write the relation for arc power (heat generated).

$$P = V I$$ where $V$ is arc voltage and $I$ is welding current. The net heat input rate is $H = \dfrac{\eta V I}{v}$ per unit length, where $\eta$ is the heat-transfer efficiency and $v$ is travel speed.

What is the difference between MIG (GMAW) and TIG (GTAW) welding?

MIG/GMAW uses a consumable wire electrode fed continuously with inert gas shielding. TIG/GTAW uses a non-consumable tungsten electrode with separate filler (if needed) under inert gas shielding, giving high-quality precise welds.

What gas mixture and approximate flame temperature are used in oxy-acetylene welding for a neutral flame?

A 1:1 ratio of oxygen to acetylene ($\ce{C2H2}$) gives a neutral flame with a temperature of about $3100$–$3300\,^{\circ}\mathrm{C}$.

Distinguish neutral, oxidizing, and carburizing oxy-acetylene flames.

Neutral flame: equal $\ce{O2}$:$\ce{C2H2}$, used for steel. Oxidizing flame: excess oxygen, hotter, used for brass/bronze. Carburizing (reducing) flame: excess acetylene, used for high-carbon steels and hardfacing.

What is the principle of resistance spot welding and which law governs heat generation?

Two metal sheets are pressed between electrodes and current is passed; heat generated at the interface (highest resistance) forms a weld nugget. Governed by Joule heating: $$Q = I^{2} R t$$ where $I$ is current, $R$ resistance, $t$ time.

What is submerged arc welding (SAW)?

An arc welding process in which the arc and weld pool are submerged under a blanket of granular flux. This shields the weld, prevents spatter, allows high deposition rates and deep penetration, used for thick sections.

What is the Heat Affected Zone (HAZ) in welding?

The HAZ is the region of base metal adjacent to the weld that is not melted but undergoes microstructural changes (grain growth, phase transformation, hardness changes) due to the thermal cycle of welding.

In welding metallurgy, why is the HAZ often the weakest or most crack-prone region in steel welds?

Rapid heating and cooling in the HAZ can produce coarse grains and hard, brittle martensite (in hardenable steels), reducing toughness and increasing susceptibility to cracking.

What is carbon equivalent (CE) and why is it important in weldability of steel?

Carbon equivalent expresses the combined hardenability effect of carbon and alloying elements: $$CE = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ A higher CE (typically $>0.4$–$0.5$) indicates greater risk of HAZ hardening and hydrogen cracking, i.e. poorer weldability.

What is hydrogen-induced (cold) cracking in welds and what factors promote it?

Cold cracking occurs in the HAZ below about $200\,^{\circ}\mathrm{C}$ after welding due to the combined presence of diffusible hydrogen, a susceptible hard martensitic microstructure, and tensile residual stress. Prevented by preheating, low-hydrogen electrodes, and post-weld heating.

Name and briefly describe four common welding defects.

Porosity: gas pockets in the weld. Slag inclusion: trapped non-metallic slag. Incomplete penetration/fusion: weld metal fails to fill the root or bond to the base. Undercut: groove melted into the base metal adjacent to the weld toe, reducing cross-section. Cracks: hot (solidification) or cold (hydrogen) cracks.