Question 5 of 7: 1080 Steel Heat-Treatment True/False; Weld Decay in 304 Stainless Piping

Question 5: 1080 Steel Heat-Treatment True/False; Weld Decay in 304 Stainless Piping (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. A eutectoid (1080) plain-carbon steel; four true/false statements about its heat-treated microstructures. A 304 austenitic stainless-steel piping system, welded, that leaks by corrosion specifically near the weld.

Find. (a) Correct/incorrect verdict and justification for each of the four statements. (b) The corrosion mechanism responsible for weld-adjacent leakage, and a practical fix that still allows welded 304 construction.

Approach

Part (a) draws on the standard TTT-diagram/heat-treatment vocabulary for a eutectoid steel: what controls pearlite hardness, how martensite actually forms (kinetics, not just product), why retained austenite occurs, and what microstructure gives maximum machinability. Part (b) is the classic "weld decay" (sensitization) failure of austenitic stainless steel: chromium-carbide precipitation at grain boundaries in the heat-affected zone (HAZ) during welding locally depletes chromium, undermining the passive film exactly in a band beside (not in) the weld metal itself.

1. (a)(i) "The hardness of pearlite is a fixed value." INCORRECT. Pearlite hardness depends on its interlamellar spacing: faster cooling through the pearlite range produces finer, more closely-spaced ferrite/cementite lamellae, which is harder and stronger (more phase-boundary area impeding dislocation motion) than coarse pearlite from slow cooling. Pearlite hardness therefore varies over a real range depending on transformation temperature/cooling rate, not a single fixed value.
2. (a)(ii) "Martensite is obtained by the isothermal transformation of austenite." INCORRECT. Martensite forms by a diffusionless, athermal shear transformation: the fraction transformed depends only on how far below $M_s$ the steel has been cooled, not on how long it is held at any one temperature. This is the opposite of an isothermal (time-dependent, diffusion-controlled) transformation like pearlite or bainite — holding a steel indefinitely at a fixed temperature between $M_s$ and $M_f$ does not produce more martensite over time; only further cooling does.
3. (a)(iii) "Retained austenite indicates the quench was too rapid." INCORRECT. Retained austenite occurs when the martensite-finish temperature $M_f$ lies below room temperature — a consequence of the steel's carbon/alloy content (which depresses both $M_s$ and $M_f$), not of how fast the quench was performed. Quenching faster does not "trap" austenite; if anything, an insufficiently fast quench risks forming pearlite or bainite instead of martensite, a different problem entirely. Retained austenite is a composition effect, not a quench-rate effect.
4. (a)(iv) "For maximum machinability the steel should be spherodised." CORRECT. Spheroidizing produces coarse, rounded (spherical) cementite particles in a soft ferrite matrix — the softest and most easily-cut microstructure available in a eutectoid steel, minimizing tool wear and cutting forces. (Machinability and strength trade off directly against each other; spheroidised steel is the softest but weakest heat-treated condition.)
5. (b) Identifying the problem: weld decay (intergranular sensitization). Type 304 is an austenitic stainless steel with a nominal carbon content around 0.08%. During welding, material adjacent to the weld (the heat-affected zone, HAZ) is held for some time in the $450^\circ$–$850^\circ$C sensitization range as heat conducts away from the weld pool. In that temperature window, carbon diffuses to and precipitates as chromium-rich carbides (Cr$_{23}$C$_6$) along the austenite grain boundaries. Because chromium diffuses far more slowly than carbon, the carbide growth strips a thin band immediately adjacent to each grain boundary of chromium, locally dropping it below the roughly 12% needed to maintain the stainless passive film. This depleted band corrodes preferentially (it is anodic relative to the chromium-rich grain interiors), producing the characteristic narrow band of intergranular attack running parallel to the weld — consistent with leaks occurring specifically near the weld, not in the weld metal itself.
6. (b) Preventing it while still using welded 304 construction. Three standard, mutually-independent fixes: (1) Use a low-carbon grade (304L, C$\le0.03$%) — too little carbon is available to form enough chromium carbide to deplete the grain boundaries significantly. (2) Use a stabilized grade (321, stabilized with Ti, or 347, stabilized with Nb) — these elements preferentially tie up carbon as titanium or niobium carbides during processing, leaving the chromium in solid solution rather than as Cr$_{23}$C$_6$. (3) Solution-anneal after welding (heat above $\approx1000^\circ$C to redissolve any chromium carbides that formed, then quench rapidly through the sensitization range) — practical for shop-fabricated components but not always feasible for an installed piping system. For new pipe fabrication, specifying 304L (or a stabilized grade) is normally the simplest, most robust fix.