Why is it not suitable to use carburizing and quenching heat treatment method for AISI1045 round steel?

From the perspective of metallurgy and practice, we discuss the following in depth:

❌ Why Carburizing and Quenching Is Not Suitable for AISI 1045 Round Steel

🔍 1. Carbon Content Is Already Too High for Carburizing

• AISI 1045 has ~0.45% carbon, which is medium-carbon steel.

• Carburizing is designed for low-carbon steels (typically ≤0.25% C), like AISI 1018 or 1020.

• Carburizing adds carbon to the surface to enable hardening — but 1045 already has enough carbon throughout to harden by itself.

📌 Problem: Carburizing 1045 can push surface carbon content too high (e.g., >0.9%), which leads to:

• Brittle martensite on the surface

• Cracking during quenching

• Reduced toughness

🔍 2. No Benefit from Case-Hardening Concept

Carburizing is meant to create:

• A hard outer shell (high-carbon martensite)

• A tough inner core (low-carbon ferrite/pearlite or tempered martensite)

🔧 But 1045 already hardens through its thickness when quenched. So carburizing doesn’t give you a hard-soft gradient — instead, you get:

• A hard shell

• A less-tough core than expected

• Possible distortion or cracking in round parts due to internal stress mismatch

🔍 3. Increased Risk of Quench Cracking

After carburizing, the high-carbon surface of 1045 will form very hard, brittle martensite upon quenching.

Combined with:

• Round bar geometry

• Internal stresses from rapid cooling

• Hardening of the core as well

→ Results in a high risk of surface or subsurface cracks.

🔍 4. Waste of Time and Cost

Since 1045 can be directly quenched and tempered, carburizing adds:

• Extra processing time (carburizing takes hours)

• Higher cost

• No performance improvement — and may even make the part worse

✅ The Right Way to Treat 1045 Round Steel

Use quenching and tempering:

1. Austenitize: 820–850°C

2. Quench: Oil (preferred) or water

3. Temper: 400–600°C depending on desired properties

This gives you:

• Uniform martensitic structure

• Hardness up to ~HRC 50

• Excellent strength, toughness, and fatigue resistance

🔄 Summary Table