Austenitic Stainless Steel Machining Rake Angle SCC

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Solution Overview

Problem

Conventional machining methods for austenitic stainless steel in nuclear power plants often result in the formation of hardened layers exceeding 300 HV in Vickers hardness, leading to stress corrosion cracking (SCC), requiring costly and time-consuming after-treatments like buffing or compressive stress application, which can be compromised by subsequent welding or other stress-inducing processes.

Innovation Solution

A machining method using a finishing-cut tool with a rake angle of +29° or more, combined with a base treatment process, to suppress surface hardening and reduce cutting resistance, thereby maintaining the hardness of the machined surface layer below 300 HV without the need for after-treatments.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Strength

If a conventional cutting tool with a rake angle of 15° or less is used to ensure edge strength and tool service life, then the tool maintains sufficient edge strength and longevity, but a hardened layer with hardness exceeding 300 HV is formed on the machined surface, leading to stress corrosion cracking (SCC)

Engineering Contradiction:

Improveedge strength of cutting toolVSAvoidhardened layer formation and SCC risk

Core Design Contradiction:

StrengthVSObject-affected harmful factors

Solution Approach 1:

The invention changes the rake angle parameter from the conventional 15° or less to +29° or more. This parameter change fundamentally alters the cutting mechanism, reducing cutting resistance and suppressing work hardening of the austenitic stainless steel surface, thereby preventing hardened layer formation with hardness exceeding 300 HV while maintaining adequate tool edge strength through the specific geometry design

Inventive Principle:

Principle #35Parameter changes

2Reliability

If after-treatments such as buffing or compressive stress application are performed to remove hardened layers and suppress SCC, then SCC occurrence is suppressed, but working hours and manufacturing costs increase

Engineering Contradiction:

ImproveSCC suppressionVSAvoidworking hours for after-treatment

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The invention converts the traditionally harmful effect of work hardening during machining into a beneficial outcome by using a positive rake angle tool that inherently suppresses hardening. The machining process itself produces a surface with hardness below 300 HV that meets SCC resistance requirements, eliminating the need for subsequent buffing or compressive stress treatments and reducing total manufacturing time

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The invention performs the SCC prevention action during the primary machining operation itself rather than as a subsequent treatment. By using a finishing-cut tool with a rake angle of +29° or more, the surface is machined to the required hardness level (below 300 HV) in the same operation that creates the final geometry, eliminating the need for separate after-treatment steps

Inventive Principle:

Principle #10Preliminary action

3Reliability

If compressive stress is applied to suppress SCC, then SCC occurrence is reduced, but subsequent welding or stress-inducing work reduces or removes the compressive stress, losing the suppression effect

Engineering Contradiction:

ImproveSCC suppressionVSAvoidcompressive stress maintenance

Core Design Contradiction:

ReliabilityVSStability of the object's composition

Solution Approach 1:

The invention establishes the desired surface hardness (below 300 HV) during the initial machining operation using a finishing-cut tool with a rake angle of +29° or more. This preliminary action creates a surface condition that inherently resists SCC without requiring subsequent compressive stress treatment, and this hardness characteristic remains stable even after welding or other stress-inducing processes are performed

Inventive Principle:

Principle #10Preliminary action

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach effectively prevents SCC occurrence during machining, reduces working hours and costs, and maintains necessary SCC resistance even after processes like welding, by ensuring the machined surface layer remains below 300 HV in Vickers hardness.

Implementation Method 1

If the rake angle is set to be as great as +29° or more, a shear angle of a chip becomes greater and a thickness of the chip becomes thinner, so that cutting force (cutting resistance) required for the cutting becomes smaller.

Methodology Applied

Scientific EffectShear angle effect:

Data Source

PatentEP2471616B1Method for machining austenite stainless steel equipment and piping, and nuclear power plant equipment and piping machined by means of said method

Publication Date: 2017.02.15 MITSUBISHI HEAVY IND LTD

AI summary

A machining method for austenite stainless steel equipment and piping, and nuclear power plant equipment and piping machined by use of this method are provided to suppress occurrence of a hardened layer due to machining, and capable of suppressing occurrence of SCC without performing an after treatment and reducing working hours and cost. The machining method for austenite stainless steel equipment and piping according to the first aspect of the present invention includes a finishing treatment process for performing a machining operation by use of a throw-away tip (5) whose rake angle is +29° or more to suppress work hardening of a surface.