Blister steel: identify the correct statement about its manufacture and characteristics for historical tool steels. Choose one accurate statement only.

Introduction / Context:
Before modern steelmaking, “blister steel” was a key intermediate for tool steels. Understanding how it is made and its microstructural features helps explain later developments such as shear steel and crucible steel.

Given Data / Assumptions:

• Blister steel originates from wrought iron exposed to carbon at high temperature.
• Historic production predates basic oxygen and electric arc routes.

Concept / Approach:

In the cementation process, bars of wrought iron are packed with charcoal and heated for prolonged periods. Carbon diffuses into the surface, creating a high-carbon outer zone and gas blisters—hence the name. The product is chemically heterogeneous and often shows fissures/cavities.

Step-by-Step Solution:

Verification / Alternative check:

Historical metallurgy texts describe conversion of blister steel to single- or double-shear steel by piling and forge welding, confirming weldability and the cementation origin.

Why Other Options Are Wrong:

Free from fissures: incorrect—blisters and heterogeneity are typical. Cannot be welded: incorrect—forge welding is precisely how shear steels were made. Electric arc route: anachronistic for blister steel.

Common Pitfalls:

Assuming modern homogeneous properties for historical steels; overlooking carburizing terminology.

Final Answer:

It is obtained by the cementation process (carburizing wrought iron).