Risks Associated with Machining Pre-Hardened PH13-8Mo

[feajob]

Hello Everyone,
I'm reaching out for your input. My background is in fatigue life assessments, not materials engineering, so I'd value your thoughts on these statements:

Risks Associated with Machining Pre-Hardened PH13-8Mo:

• Increased Residual Stresses:
  
  • Machining hardened material generates higher cutting forces and heat, leading to significant residual stresses. These stresses can be tensile, potentially initiating or propagating cracks, especially in fatigue-sensitive applications.
  • These residual stresses can also lead to distortion during subsequent heat treatments (if any) or in service.
• Reduced Fatigue Life:
  
  • The combination of residual stresses and the inherent stress concentrations from machining operations can significantly reduce fatigue life.
  • The surface integrity, critical for fatigue performance, can be compromised by the higher machining forces.
• Increased Risk of Cracking:
  
  • PH13-8Mo, while strong, can be susceptible to cracking if machined aggressively in the hardened state.
  • Existing microcracks or imperfections can propagate more readily under the higher stresses induced by machining.
• Difficulty in Achieving Tight Tolerances:
  
  • Hardened materials are more difficult to machine accurately.
  • This can make it challenging to achieve tight tolerances and surface finishes, potentially affecting the functional performance of the part.
  • Springback is also a much larger issue.
• Tool Wear and Breakage:
  
  • Machining hardened steel results in significantly higher tool wear and an increased risk of tool breakage. This leads to increased costs and potential surface damage to the workpiece.
• Heat Generation:
  
  • Excessive heat build up during machining can alter the material properties of the steel, and create unwanted localized hardness changes.
• Stress Corrosion Cracking (SCC) Potential:
  
  • Residual stresses can increase the susceptibility of PH13-8Mo to stress corrosion cracking in corrosive environments.

Why Solution Annealing is Preferred:

• Solution annealing softens the material, making it much easier to machine.
• It minimizes residual stresses, reducing the risk of distortion and cracking.
• It allows for better control of tolerances and surface finishes.
• The final heat treatment after machining provides the desired strength and hardness without the detrimental effects of machining in the hardened state.

In summary: Machining PH13-8Mo in the pre-hardened condition introduces significant risks to the structural integrity of the part. While it might seem like a time-saving measure, the potential for reduced fatigue life, cracking, and distortion far outweighs any perceived benefits. Therefore, adhering to the recommended practice of machining in the solution-annealed condition (as per our part list) and heat treating afterward is crucial for ensuring the reliable performance of PH13-8Mo components.

Do you agree with the above statements? If not, please provide details so I can improve my understanding.
Thank you,
Ali

 

[mfgenggear]

OP
all I have is my experience in this matter.
it is common to machine precipitation steels
never had any fall out or any notes
on any of my customer drawing preventing
us.
harden vs annealing
it has been my experience when machine dead soft steel. it is gummy, produces terrible surface finish. and does not form
a proper machining because it tends to tear.
instead cutting properly.

so if the higher hardness is required
rouch machine, heat treat then finish
machine close tolerance.
every part has to be evaluated case by case.
and it situation has its challenges

data sheet
-8 PH stainless steel, also known as UNS S13800 or PH 13-8 Mo, is a martensitic precipitation-hardening stainless steel with excellent strength, high hardness, superior toughness, and good corrosion resistance, particularly in the fully hardened condition.

Here's a more detailed overview:


Key Characteristics:

• Type: Martensitic, precipitation-hardening stainless steel.
  
  
• Strengths: High strength, high hardness, good corrosion resistance, and superior toughness.
  
  
• Applications: Aerospace components, petrochemical industries, power generation, and nuclear applications.
  
  
• Heat Treatment: Can be age-hardened by a single low-temperature treatment.
  
  
• Corrosion Resistance: Similar to 304 stainless, with the greatest resistance in the fully hardened condition.
  
  
• Oxidation Resistance: Excellent up to 1100°F (539°C).
  
  
• Stress Corrosion Cracking: This alloy has good resistance to stress corrosion cracking.
  
  
• Fabrication: Good fabrication characteristics.
  
  
• Melting: Double melted in a Vacuum Induction Furnace followed by Consumable Electrode Vacuum Melt (VIM-VAR).
  
  
• Common Specifications: AMS 5629, AMS 5864, ASTM A564 (XM-13), ASTM A693 (XM-13), ASTM A705 (XM-13).
  

Mechanical Properties (Condition H1000):

• Ultimate Tensile Strength: 205 ksi.
• 0.2% Offset Yield Strength: 190 ksi.
• Elongation in 2": 10%.
• Reduction of Area: 50%.
  

Heat Treatment Conditions:

• Condition A:
  Solution heat treated at 1675-1725°F (927°C) for 15-30 minutes, then air cooled or oil quenched to below 60°F (16°C).
• H950, H1000, H1025, H1050, H1100, H1150:
  Aging temperatures ranging from 950-1150°F (510-620°C) for 4 hours, then air cooled.
• H1150M:
  Soak solution treated material at 1400°F (760°C) for 2 hours, air cool, then re-heat to 1150°F (620°C) for 4 hours and air cool.

 

[mfgenggear]

OP
As a manufacture I would do carefully controls
Feeds and speeds as to reduce stress induced in machining. Many operations I would add a stress relief after aggressive machining .
Rough machine stress relief, then semi finish turn , then finish grind precision close tolerances.
I was involved with thousands parts. Unfortunately
This ia learned the hard knocks with experience.
But this is the job of manufacturing engineering , QC and the shop. And their expertise. As designer
It is important to understand it can be designed
And fabricated. And they have to their job.

 

[mfgenggear]

One more note. Before I retired QpQ, iso9001,
And risk assessment was part of our duties.
But this has been my duties even before implementing these specification and procedures. I would do risk assessment to prevent discrepant parts and issue with all operation. During fabrication all machining operations and qc inspection was recorded.
For review. All red line changes incorporated.

 

[EdStainless]

I wouldn't make many of those statements unless I knew what strength level we are talking about.
At H1150, H1100, and maybe even H1050 the machining isn't that big of a deal.
If the material is H950 or H1000 the strength and hardness could be problem unless you are experienced with hard machining.
The biggest problem with machining as annealed and then aging is the size change.
Yes it is predictable, but you still have to account for it.
If I wanted the easiest machining and tolerance control I would HT to H1150M (over aged, 1400+1150) and then machine.
You would follow this with a re-anneal and the age to the desired strength.
All of the 13-8 that I ever worked with as re-melted material and made to AMS specifications.
The AMS HT requirements are strict.
The widest latitude that you are allowed for temperatures is +/-25F, and some are tighter.

 

[mfgenggear]

I would have to look it up in the AMS heat treat spec but it gives the equivalent HRc hardness.
I can say this 40 Hrc can readily be machined with no issues. There is a trend for exotic and precipitation harden steel to be hard turn as been done for many decades. As long as the machinery has the correct rigidity and horse power. And correct tooling.

 

[feajob]

Hi Guys,
I appreciate your replies. I'd like to provide additional information: The original material drawing called for PH13-8Mo stainless steel, solution treated according to AMS 5629, with a subsequent heat treatment to condition H1000 (205 KSI minimum UTS) per AMS 2759/3. We received material that was solution treated and heat treated to Cond. H1000 per AMS-H-6875.
Please advise if these information alter your earlier conclusions.
Thanks,
Ali

 

[mfgenggear]

Hi Guys,
I appreciate your replies. I'd like to provide additional information: The original material drawing called for PH13-8Mo stainless steel, solution treated according to AMS 5629, with a subsequent heat treatment to condition H1000 (205 KSI minimum UTS) per AMS 2759/3. We received material that was solution treated and heat treated to Cond. H1000 per AMS-H-6875.
Please advise if these information alter your earlier conclusions.
Thanks,
Ali

Not at all

 

[mfgenggear]

All these hardness can be machine but if stress is a concern, rough turn then precipitation harden then semi or finish machine depending on the tolerances.
In the H1000 condition, 13-8 PH stainless steel typically exhibits a Rockwell C hardness of around 43.

Here's a more detailed breakdown:

• 13-8 PH Stainless Steel:
  This is a precipitation-hardened (PH) stainless steel alloy known for its high strength and good corrosion resistance.
  
  
• H1000 Condition:
  This refers to a specific heat treatment condition where the material is aged at 1000 °F (538 °C).
  
  
• Rockwell C Hardness:
  The H1000 condition of 13-8 PH stainless steel generally yields a Rockwell C hardness of around 43.
  
  
• Other Conditions:
  The hardness can vary depending on the heat treatment condition. For example, H950 condition has a Rockwell C hardness of 45, and H1025 condition has a Rockwell C hardness of 41.

 

[EdStainless]

The correct specification is AMS 5629/H1000.
If you are buying hardened material there is no point in listing the HT spec separately.
It is now baked into the specifications.
You also need to be aware that there is a minimum and maximum on the strength in these specs.
If you are over the max strength you are to re-age at +25F from the original aging treatment.

 

[EdStainless]

I'll add that the AMS specs do not have hardness limits.
It is all strength based.

 

[mfgenggear]

I'll add that the AMS specs do not have hardness limits.
It is all strength based.

Agreed buy I use the equivalent hardness for evaluation. Hardness is easier for evaluation.
Plus I always take hardness check to make sure.