---
title: "Weldability and embrittlement of steels"
subject: "Material Science"
chapter: 7
key: "semester_3/material_science:weldability-and-embrittlement-of-steels"
source: https://www.bctengineer.com/q/semester_3/material_science/weldability-and-embrittlement-of-steels
site: BCT Engineering
---

# Weldability and embrittlement of steels

Material Science · Chapter 7: Types of steels and cast iron

- Asked in: year not given, 5 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/weldability-and-embrittlement-of-steels
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-7

## Question

Explain the weldability of steels and the factors affecting it. Describe temper embrittlement and hydrogen embrittlement of steels.

## Short answer

Weldability depends on carbon content and alloying; carbon equivalent <0.4 is weldable without preheat. Temper embrittlement: impurity segregation at grain boundaries in 375–575°C range, reduced by Mo or rapid cooling. Hydrogen embrittlement: atomic hydrogen causes delayed cracking in high-strength steels, remedied by baking.

## Answer

#### Weldability of steels
Weldability is the ease with which a steel can be welded to give a sound joint of adequate strength and toughness. Poor weldability arises from the hard, brittle **martensite** that forms in the heat-affected zone (HAZ) on rapid cooling.

Factors affecting weldability:
- **Carbon content**: below 0.25% C is readily weldable; above 0.45% needs preheating and post-weld heat treatment.
- **Alloying elements**: Mn, Cr, Mo, Ni raise hardenability, increasing the risk of cracking. This is expressed by the carbon equivalent:

$$
CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}
$$

Steels with CE below about 0.4 are generally weldable without preheat.
- **Thickness and cooling rate**: thick sections cool faster.
- **Hydrogen**, sulphur and phosphorus: promote cracking and hot shortness.
- Remedies: preheating, low-hydrogen electrodes, controlled heat input, post-weld heat treatment.

#### Embrittlement
- **Temper embrittlement**: alloy steels (containing Ni, Cr, Mn) held or slowly cooled between about 375 and 575 °C lose toughness, because impurities (P, Sb, Sn, As) segregate to prior-austenite grain boundaries, causing intergranular fracture. Remedies: add Mo (about 0.5%), keep impurities low, quench rapidly through the range.
- **Hydrogen embrittlement**: atomic hydrogen absorbed during pickling, plating or welding diffuses into the steel and reduces ductility and causes delayed cracking at stress well below yield. High-strength steels are most susceptible. Remedy: baking (about 200 °C), low-hydrogen processes.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What mainly causes poor weldability in high-carbon steels?
   A. Hard, brittle martensite in the heat-affected zone from rapid cooling
   B. Low melting point
   C. Too much ferrite
   D. Too much pearlite
   Answer: A. Fast cooling of the HAZ forms martensite, which is prone to cracking. (Key point: Cause of poor weldability)

2. What causes temper embrittlement of alloy steels held at 375-575 °C?
   A. Hydrogen diffusing into the steel
   B. Formation of retained austenite
   C. Carbon dissolving into ferrite
   D. Segregation of P, Sb, Sn, As to prior-austenite grain boundaries
   Answer: D. Impurities at grain boundaries cause intergranular fracture; Mo addition and low impurity levels help. (Key point: Temper embrittlement)

3. What is a standard remedy for hydrogen embrittlement?
   A. Quench rapidly through 375-575 °C
   B. Baking at about 200 °C and using low-hydrogen processes
   C. Adding more carbon
   D. Adding Mo
   Answer: B. Baking drives out absorbed hydrogen; low-hydrogen electrodes and processes prevent its entry. (Key point: Hydrogen embrittlement remedy)

## Related in Material Science

- [Annealing of cold-worked metals](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aannealing-recovery-recrystallization-grain-growth) — Material Science, chapter 4

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