Improvement treatments

Describe the hardening process of a steel (approximate temperature, cooling medium and resulting microstructure) and explain what properties it gains and what drawback it introduces.

The figure shows the cooling curves of two identical steel components: one is quenched in water and the other is left to cool inside the switched-off furnace. Indicate which curve corresponds to hardening and explain what difference in hardness you expect between both components.

After hardening, steels usually undergo tempering. Explain what tempering consists of, what problem from hardening it corrects, and why an untempered hardened tool would be dangerous in service.

Describe annealing: to what temperature the steel is heated, how it is cooled, and what effect it has on its hardness and internal structure. In what workshop situation is it applied before further working a component?

Normalising cools in still air instead of inside the furnace. Compare annealing and normalising: which is faster and cheaper, which leaves the steel somewhat harder, and what normalising is used for after forging.

Order these cooling media from highest to lowest severity (cooling rate): oil, air, brine (salt water). Explain the relationship between the severity of the medium, the achievable hardness, and the risk of cracking or distortion.

Explain the basic idea of carburising: which element diffuses into the surface of the steel, at approximately what temperature it is performed, and why, after carburising, the component must be hardened and tempered.

Compare these three surface treatments: carburising, nitriding and hard chrome plating. For each one, indicate what is added or modified on the surface, a typical application, and why it is beneficial to harden only the outer layer rather than the entire component.

Compare a thermochemical treatment (carburising: modifies surface composition) with a hard PVD-type coating (deposited titanium nitride layer): typical thickness, adhesion, process temperature, and an application where each is preferred (gears versus drill bits).

TTT (time-temperature-transformation) diagrams show when the transformation of austenite starts and finishes upon cooling. Qualitatively explain why very rapid cooling "bypasses" the curves and produces martensite, whereas slow cooling passes through them and produces pearlite, which is softer.

Define the hardenability of a steel and explain why a carbon steel only hardens in thin layers whereas an alloy steel (with chromium or molybdenum) through-hardens. What practical consequence does this have when choosing the steel for a thick shaft?

After hardening, a long component has warped and another shows microcracks. Explain what residual hardening stresses are (cooling difference between surface and core), how they cause distortion, and what process measures reduce them (uniform heating, less severe cooling media, component design).

A factory manufactures 1.000 drive shafts of 8,5 kg each per month. To ensure wear resistance on the bearing supports and cams, the production department compares two alternatives:\n- Option A: Heating the entire part in a continuous gas furnace for 3,5 h at 870 °C, followed by full quenching and tempering in an electric furnace (equivalent energy consumption: 12,0 kWh per part).\n- Option B: Surface induction hardening localized exclusively on the bearing supports using a high-frequency inductor for 8 s with integrated spray quenching and self-tempering, keeping the core in its pre-normalized state (consumption: 1,4 kWh per part).\na) Justify from the mechanical behavior standpoint why Option B is preferable to Option A for a component subjected to fatigue and torsion.\nb) Calculate the annual energy savings (in kWh and percentage) that adopting Option B over Option A would entail.\nc) An environmental report states that "the greenest heat treatment is the one that consumes no energy, so all heat treatment should be eliminated." Refute this statement by evaluating the life cycle environmental impact of the steel if the absence of treatment reduces the part's service life to one quarter.

In an industrial press, deep cylindrical casings are drawn from mild steel sheet (0,10 % C). The process requires two successive deep drawing strokes. When the two operations are carried out consecutively without an intermediate stage, the sheet tears systematically on the second stroke, resulting in 100 % defective parts. The testing laboratory extracts a specimen after the first stroke and verifies that the yield strength has increased from 210 MPa to 550 MPa, while the elongation at break has dropped from 35 % to 3,5 %.\na) Interpret the tensile test results, explaining at the microstructural level the physical mechanism that has altered the steel's properties.\nb) Propose the specific heat treatment that must be inserted between both forming operations, justifying the appropriate operating temperature with respect to the critical points of the Fe-C diagram and the transformation experienced by the crystal grains.\nc) If incorporating the furnace for the intermediate treatment consumes 0,40 kWh per part (at a cost of 0,15 €/kWh) and each casing scrapped due to tearing represents a material and processing loss of 3,60 €, calculate above what percentage of defect reduction turning on the furnace becomes economically profitable.

A batch of carbon steel threaded pins (0,50 % C) is heated to 850 °C in a muffle furnace with atmospheric air for 2 h before being quenched in oil. After tempering, the surface hardness test yields 32 HRC on the surface of the screw threads, but when sectioning the part and measuring at a depth of 0,6 mm, 52 HRC is obtained. Furthermore, the threads exhibit a rough scale that ruins the metric tolerances.\na) Explain, using the corresponding chemical reactions with the gases in the air, why this drop in surface hardness and the formation of scale at high temperature have occurred.\nb) Predict what mechanical failures the pin will suffer if it enters service installed in a vibrating machine subjected to alternating loads.\nc) Compare technically and economically two furnace solutions to prevent these two problems: controlled atmosphere furnaces (reducing or inert gas) versus vacuum furnaces, justifying which one you would choose if the production were high-volume with strict dimensional requirements.