Exercises: Improvement treatments

Explain what a heat treatment is and give three examples with their general objective: which one hardens, which one softens to facilitate machining, and which one reduces brittleness after hardening.

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.

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.

Distinguish between through-heat treatment (affects the entire component) and surface treatment (only the outer layer). Give two examples of each and explain why a gear tooth usually requires only surface treatment.

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.

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.

A brushcutter blade dulls quickly and also suffers impacts against stones. Propose a combination of heat treatments for the steel of the blade and justify it: what hardness the cutting edge needs, what toughness the assembly needs, and how both are achieved.

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 suspension spring requires hardness but, above all, elasticity and fatigue strength. Propose its treatment sequence (hardening and medium-temperature tempering) and explain why low-temperature tempering (maximum hardness) would be counterproductive in a spring.

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).

In the development workshop of a light electric vehicle, a technician proposes manufacturing an alloy steel transmission shaft by first machining the final dimensions (including keyways and bearing seats) and then applying through hardening and tempering to the entire part. He claims that "with full hardening the part will resist both wear on the bearing supports and torsional stresses."\na) Refute the technician's proposal by justifying two serious problems that will arise: one dimensional and geometric during the treatment, and another mechanical in service under torsion and cyclic loads.\nb) Design a complete and orderly alternative roadmap for manufacturing the shaft (forming or machining stages, bulk heat treatments, and localized surface treatment).\nc) Justify each designed stage by relating it to the target microstructure and the mechanical property it imparts to the shaft.

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.

A batch of 500 carbon steel (0,80 % C) stonecutting chisels was treated in the workshop by heating them to 880 °C and quenching them directly in a brine water bath at 15 °C. Upon removal from the bath, 30 % of the chisels exhibit visible cracks along the cutting edge. Furthermore, when workers test the apparently intact chisels on site, the cutting edge chips and fractures on the very first impact.\na) Analyze the metallurgical causes of both failures (the immediate cracks after the bath and the brittle fracture on first use), explaining which crystalline phases and stress states were generated.\nb) Identify at least two critical errors made in the thermal and cooling parameters of the process.\nc) Design the corrected thermal procedure (approximate austenitizing temperature, suitable cooling medium, and subsequent heat treatment with its temperature range) so that the chisels exhibit high hardness at the cutting edge along with the toughness required to withstand the hammer blow.

To cut costs in the manufacturing of gearbox pinion gears subjected to severe dynamic impacts and contact wear on the teeth, a company proposes replacing a low-carbon steel (0,15 % C) with a thermochemical carburizing treatment with a through-quenched and tempered hypereutectoid steel (0,95 % C) at 180 °C.\na) Predict the mechanical behavior and the failure mode that the 0,95 % C gear will suffer during abrupt gear shifting in the vehicle, comparing it with the carburized gear.\nb) Design the complete manufacturing route for the carburized pinion gear (selection of steel grade, machining, thermochemical stage, quenching, tempering, and finishing), justifying the microstructural function of each stage.\nc) Describe accurately the qualitative profile of hardness (HRC) and carbon concentration (% C) from the tooth surface to the center of its core, justifying why this variation exists.

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.

During electric arc welding of two medium-carbon steel structural profiles (0,45 % C), subsequent inspection reveals the formation of longitudinal brittle cracks in the Heat-Affected Zone (HAZ), right next to the weld bead, while the filler metal shows no defects. A micrographic test shows that in the HAZ the microstructure has changed from ferritic-pearlitic to high-hardness martensite (59 HRC).\na) Explain how quenching could occur in the HAZ if the joint was allowed to cool naturally in ambient air without being immersed in water or oil.\nb) Analyze the physical origin of the residual stresses that, combined with the martensite, caused the underbead cracking.\nc) Design an operational strategy to perform the welding without this fracture occurring, metallurgically justifying the role of preheating the parts and of a post-weld stress-relief heat treatment.

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.

In a machining workshop, a batch of tool steel drill bits that have just been quenched and are at 65 HRC has been acquired. To save the cost of an external treatment, they propose performing the tempering in the domestic electric oven in the workshop kitchen, setting the thermostat to 250 °C for 60 minutes. As the technical manager, you suspect that the limitations of this appliance could ruin the batch of tools.\na) Justify two critical deficiencies that a domestic oven presents compared to an industrial heat treatment furnace regarding temperature control and heat transfer.\nb) Design an experimental protocol to evaluate the viability of the oven before risking the entire batch: define what variables you will measure, what auxiliary instruments you will need (beyond the oven thermostat), how you will distribute the test specimens inside the chamber, and what mechanical tests you will perform.\nc) Establish the quantitative criteria for hardness dispersion (HRC) and thermal tolerance that will conclusively determine whether the oven is suitable or must be rejected.