Question 5 of 7: 1080 Steel Statements; Weld Decay in 304 Stainless Steel

Question 5: 1080 Steel Statements; Weld Decay in 304 Stainless Steel (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) Four statements about 1080 (eutectoid, 0.80%C) steel. (b) 304 stainless steel pipe, welded, corroding and leaking near the weld while carrying a corrosive liquid.

Find. (a) True/false with justification for each statement. (b) Identify the corrosion mechanism and the prevention steps.

Approach

Part (a) tests whether each statement correctly describes the underlying transformation physics of a eutectoid steel (pearlite formation, martensite formation, retained austenite, and spheroidizing) rather than requiring calculation. Part (b) is the classic weld-decay (sensitization) problem for austenitic stainless steel: identify the Cr-carbide precipitation mechanism at the heat-affected zone, then apply the three standard industrial remedies.

1. (i) "The hardness of pearlite is a fixed value" — INCORRECT. Pearlite's hardness depends on its interlamellar spacing, which is set by the transformation (cooling) rate: fine pearlite (formed closer to the TTT-diagram nose, faster cooling) has much more ferrite–cementite phase-boundary area per unit volume than coarse pearlite (formed near $A_1$, slow cooling), and that boundary area obstructs dislocation motion more effectively, so fine pearlite is measurably harder than coarse pearlite from the same steel.
2. (ii) "Martensite is obtained by the isothermal transformation of austenite" — INCORRECT. Martensite forms by a diffusionless, athermal (shear-type) transformation during rapid continuous cooling below $M_s$ — the transformed fraction depends on how far below $M_s$ the steel is cooled, not on holding time at a fixed temperature. It is bainite, not martensite, that is the classic isothermal transformation product on a TTT diagram.
3. (iii) "Retained austenite indicates the quench was too rapid" — INCORRECT. Retained austenite occurs because the martensite-finish temperature $M_f$ lies below room temperature (common in higher-carbon steels like 1080, since carbon depresses both $M_s$ and $M_f$) — some austenite simply never transforms on cooling to room temperature, regardless of how fast the quench is. A slower quench would not eliminate it (and risks missing the nose and forming pearlite instead); what actually converts retained austenite to martensite is a deep sub-zero (cryogenic) treatment.
4. (iv) "For maximum machinability the steel should be spherodised" — CORRECT. A spheroidized microstructure (globular cementite particles in a soft ferrite matrix, produced by a long sub-critical anneal) is the softest and most machinable condition available for a given carbon content — it minimizes the cutting resistance presented by the harder lamellar (pearlite) or acicular (martensite) microstructures, which is why spheroidizing is the standard pre-machining treatment for high-carbon steels such as 1080.
5. (b) Identify the problem: weld decay (intergranular corrosion) from sensitization. During welding, the heat-affected zone adjacent to the weld spends time in the $450$–$850^\circ$C sensitization range. In that range, chromium diffuses to the grain boundaries and combines with carbon to precipitate $\text{Cr}_{23}\text{C}_6$ carbides, which locally strips the immediately adjacent grain-boundary regions of chromium — often below the $\sim12\%$ Cr needed to sustain the passive oxide film. The result is a narrow, Cr-depleted band along the grain boundaries in the HAZ (adjacent to, not within, the weld metal itself), which corrodes preferentially in the corrosive service liquid: exactly the leak pattern described ("near the weld").
6. Prevention while still using welded 304. Three standard remedies:
   1. Post-weld solution anneal — reheat the fabricated assembly above $\sim1050^\circ$C and rapidly quench, which redissolves the grain-boundary carbides and homogenizes chromium back across the boundary. This is the most direct fix given the piping is already fabricated from standard 304.
   2. Switch to a low-carbon grade (304L) on future fabrication, so there is too little carbon available to precipitate a significant amount of $\text{Cr}_{23}\text{C}_6$ during the weld thermal cycle.
   3. Switch to a stabilized grade (321 with Ti, or 347 with Nb), where the stabilizing element preferentially forms TiC/NbC instead of chromium carbide, protecting the grain-boundary chromium.