ROOT CAUSE FAILURE ANALYSIS
1. Scope.
This failure analysis is perform to known of root cause of crack in part of “Pressure Reducing Bushing”. This part was made from material Martensitic Stainless steel of the 420 series.
2. Martensitic Stainless steel (ASM Vol-9 “Metallography and Microstructures” P.285)
The martensitic stainless steel of the 400 series can be heat-treated to a wide range of useful hardness and strength level. These steel contain of chromium as the major alloying element. Chromium and carbon content are balance so that the soft Austenitic phase which develops at high temperatures transform to the hard martensitic phase during cooling to room temperature.
The Martensitic stainless steel are more sensitive to heat treating variables than are carbon and low alloys steels, rejection rates due to faults in heat treating are correspondingly high.
Heat treatment Process :
Cleaning, To avoid contamination, all part and heat treating fixture must be clean thoroughly before they are placed in the furnace. Proper cleaning is particularly importance when the heat treatment is to be performed in a protective atmosphere. Grease, oil, and even location lines made by an ordinary lead pencil can cause carburization. Perspiration stain from fingerprints are source of chloride contamination and may cause severe scaling in oxidizing atmospheres. Furthermore, a protective atmosphere can not be effective unless it is permitted to make unobstructed contact with metal surfaces.
Pre heating, Martensitic stainless steel normally are hardened by being heated to the Austenitizing range of 925 to 1065 deg. Celsius. And then cooled in air or oil.
The thermal conductivity of stainless steel is characteristically lower than that of carbon and alloys steel. Accordingly, high thermal gradient and high stresses during rapid heating may cause warp age and cracking in some part. To avoid these problem, preheating is usually recommended in the treatment of martensitic stainless steel.
In annealing or hardening, the following parts should be preheated. :
· Heavy section part
· Part with both thin and thick sections
· Part with sharp corners and reentrant angels
· Part machined with heavy deep cuts, etc.
Pre heating is normally accomplished at 760 to 790 deg. C. and heating need be continued only long enough to ensure that all portions of each part have reached the pre heating temperature.
Austenitizing Temperatures, soaking times, and quenching media for AISI 420 are :
Austenizing Temperature (o C)
Quenching Medium *)
Tempering Temperature (oC)
Hardness (HRC)
980 – 1065
Air or Oil
205 – 370
48 - 56
Soaking time employed in the hardening of martensitic stainless steel represent a compromise between achieving maximum solution of chromium-iron carbides for maximum strength and corrosion resistance, and avoiding decarburization, excessive grain growth, retained autenite, brittleness, and quench cracking. For section 13 mm thick and under, a soaking time of 30 to 60 minute is sometimes recommended.
Quenching, Martensitic Stainless steel can be quench in either oil or air, some decrease in corrosion resistance and ductility, resulting from air quenching, may accour in this grade. These steel may precipitate carbide in grain boundary area if heavy section are cooled slowly trough temperature range of about 870 to 540 oC. Although oil quenching is preferred, air cooling may be required for large or complex section to prevent distortion or quench cracking.
Quench Cracking. (Metal Handbook V.11)
Quench crack in steel result from stresses produced during the Austenite to martensite transformation, which is companied by an increase in volume. As quenched martensite is hard and exhibits almost no ductility.
When a component made of fully harden able alloy steel is quenched, martensit form first at the outermost surface, which are first to reach the martensite start, Ms temperature. The martensitic expansion work the softer austenite below and is almost unrestricted in its growth at the outer surface. As cooling progress and the material near the center of the section reaches the Ms temperature, the expansion accompanying the newly formed martensite is restrict by the outer layer of martensite form earlier. This result in internal stress that places the surface in tension. Cracking occurs when enough martensite has formed to set up an internal stress sufficient to exceed the tensile strength of the as quenched martensite at the outer surface of the component.
Quench crack have characteristics that are easily recognized, for example :
· First the generally run from the surface toward the center of mass in relatively straight line. The crack is also likely to open or spread and may exhibit a shear lip at the extreme surface.
· Second, because quench cracking occur at relatively low temperature, the crack will not exhibit any decarburization when examined macroscopically or microscopically.
· The fracture surface will exhibit a fine crystalline texture. When tempered after quenching, the fracture surface may be blackened by oxidation.
Factor Controlling Cracking. Any condition that concentrates the stresses encountered in quenching will promote the formation of quench crack. Whenever possible, sharp change in section, such as rectangular key ways or holes, should be avoid during quenching. Cold stamping marks use to identify part have also been know to nucleate quench crack.
The selection of a suitable quenching medium is often a significant factor in eliminating quench carack.
The quenchants most commonly used are caustic solution, brine, water, oil, and air. The fastest quench can be obtained in caustic solution and the slowest is air.
3. Back ground
These part of Pressure reducing bushing was made from AISI-420 material, after heat-treatment and machining process as engineering drawing, there was found defect (crack) in all areas of holes at flange such as in drawing bellow :
The crack are appears when these part was sent to customer after at about 24 hours, it indication as Delay Cracking.
3. Step of process production :
Raw Material
Roughness Machining
Heat-Treatment
NDT and Hardness Inspection
Final Machining
Final Inspection (NDT & Hardness)
Delevery
Rejected by Customer
4. Heat-Treatment
Heat-treatment was conduct as below, these process to achieve of Hardness 40 – 50 HRC
4.1. Hardening.
1 Hr
1 Hr
1 Hr
Oil Quench
Time
Temp (oC)
1000
760
540
4.2. Tempering
Temp (oC)
1 Hr
Time
540
5. Step of Analysis
Here are step of analysis as below :
Part Reject
DATA :
- Chemical Composition
- Micro Structure
- Hardness
Analysis and Conclusion
6. Data
6.1. Chemical composition was conduct by “Atomic Emission Spectrometer / Spark” method.
Element
Content (%)
C
0.75672
Si
0.22644
Mn
0.81994
P
0.02824
S
0.007
Ni
0.54478
Cr
15.40220
Mo
0.95183
V
0.10340
Cu
0.06945
6.2. Micro structure and Hardness was conduct as (see attachment)
- The material before Heat-treatment are about 25 HRC
- Hardness after Heat-treatment are 60 HRC
7. Analysis and Evaluation
7.1. Chemical Composition.
From data laboratory that materials was made from AISI-420 or Martensitic Stainless steel, but there is Carbon content are to High (=0.75 %C), the martensitic stainless steel AISI 420 commonly consist of , carbon are 0.15 to 0.20, as indicated to code 420, that mean :
4 XX are series of Martensitic
X 20 are Carbon content 0.15 to 0.20 %
7.2. Microstructure
- After Heat-treatment the micro structure are consist of Ferit in a matrix Martensite tempered and carbide.
- There is some crack was found in areas of ferrite phase.
7.3. Hardness are 60 HRC
7.4. Dimension
- In these part are different thickness (thick wall) in hole area
- In the area of holes there are not Radius.
Conclusion :
- The carbon content is too high so there caused form of Martensit and the excessive carbide will formed in the grain, as look like in the fig.1.
- From the micro structure as indicated in fig 1, there are ferrite phase in a matrix Matensite and carbides disperse between it. The martensite and carbide are very hard but Ferrite is soft and weak soo the presence of ferrite is initiate a Crack.
- In the Quenching process the martensit has internal stress and Tempering is mean to release it but if the temperature and holding time is too high, the structure be comes ……
- The achieved of hardness is too high (60 HRC) for this part (with the design of part), it may be enough 50 HRC.
- The difference Thickness of the hole areas and
This failure analysis is perform to known of root cause of crack in part of “Pressure Reducing Bushing”. This part was made from material Martensitic Stainless steel of the 420 series.
2. Martensitic Stainless steel (ASM Vol-9 “Metallography and Microstructures” P.285)
The martensitic stainless steel of the 400 series can be heat-treated to a wide range of useful hardness and strength level. These steel contain of chromium as the major alloying element. Chromium and carbon content are balance so that the soft Austenitic phase which develops at high temperatures transform to the hard martensitic phase during cooling to room temperature.
The Martensitic stainless steel are more sensitive to heat treating variables than are carbon and low alloys steels, rejection rates due to faults in heat treating are correspondingly high.
Heat treatment Process :
Cleaning, To avoid contamination, all part and heat treating fixture must be clean thoroughly before they are placed in the furnace. Proper cleaning is particularly importance when the heat treatment is to be performed in a protective atmosphere. Grease, oil, and even location lines made by an ordinary lead pencil can cause carburization. Perspiration stain from fingerprints are source of chloride contamination and may cause severe scaling in oxidizing atmospheres. Furthermore, a protective atmosphere can not be effective unless it is permitted to make unobstructed contact with metal surfaces.
Pre heating, Martensitic stainless steel normally are hardened by being heated to the Austenitizing range of 925 to 1065 deg. Celsius. And then cooled in air or oil.
The thermal conductivity of stainless steel is characteristically lower than that of carbon and alloys steel. Accordingly, high thermal gradient and high stresses during rapid heating may cause warp age and cracking in some part. To avoid these problem, preheating is usually recommended in the treatment of martensitic stainless steel.
In annealing or hardening, the following parts should be preheated. :
· Heavy section part
· Part with both thin and thick sections
· Part with sharp corners and reentrant angels
· Part machined with heavy deep cuts, etc.
Pre heating is normally accomplished at 760 to 790 deg. C. and heating need be continued only long enough to ensure that all portions of each part have reached the pre heating temperature.
Austenitizing Temperatures, soaking times, and quenching media for AISI 420 are :
Austenizing Temperature (o C)
Quenching Medium *)
Tempering Temperature (oC)
Hardness (HRC)
980 – 1065
Air or Oil
205 – 370
48 - 56
Soaking time employed in the hardening of martensitic stainless steel represent a compromise between achieving maximum solution of chromium-iron carbides for maximum strength and corrosion resistance, and avoiding decarburization, excessive grain growth, retained autenite, brittleness, and quench cracking. For section 13 mm thick and under, a soaking time of 30 to 60 minute is sometimes recommended.
Quenching, Martensitic Stainless steel can be quench in either oil or air, some decrease in corrosion resistance and ductility, resulting from air quenching, may accour in this grade. These steel may precipitate carbide in grain boundary area if heavy section are cooled slowly trough temperature range of about 870 to 540 oC. Although oil quenching is preferred, air cooling may be required for large or complex section to prevent distortion or quench cracking.
Quench Cracking. (Metal Handbook V.11)
Quench crack in steel result from stresses produced during the Austenite to martensite transformation, which is companied by an increase in volume. As quenched martensite is hard and exhibits almost no ductility.
When a component made of fully harden able alloy steel is quenched, martensit form first at the outermost surface, which are first to reach the martensite start, Ms temperature. The martensitic expansion work the softer austenite below and is almost unrestricted in its growth at the outer surface. As cooling progress and the material near the center of the section reaches the Ms temperature, the expansion accompanying the newly formed martensite is restrict by the outer layer of martensite form earlier. This result in internal stress that places the surface in tension. Cracking occurs when enough martensite has formed to set up an internal stress sufficient to exceed the tensile strength of the as quenched martensite at the outer surface of the component.
Quench crack have characteristics that are easily recognized, for example :
· First the generally run from the surface toward the center of mass in relatively straight line. The crack is also likely to open or spread and may exhibit a shear lip at the extreme surface.
· Second, because quench cracking occur at relatively low temperature, the crack will not exhibit any decarburization when examined macroscopically or microscopically.
· The fracture surface will exhibit a fine crystalline texture. When tempered after quenching, the fracture surface may be blackened by oxidation.
Factor Controlling Cracking. Any condition that concentrates the stresses encountered in quenching will promote the formation of quench crack. Whenever possible, sharp change in section, such as rectangular key ways or holes, should be avoid during quenching. Cold stamping marks use to identify part have also been know to nucleate quench crack.
The selection of a suitable quenching medium is often a significant factor in eliminating quench carack.
The quenchants most commonly used are caustic solution, brine, water, oil, and air. The fastest quench can be obtained in caustic solution and the slowest is air.
3. Back ground
These part of Pressure reducing bushing was made from AISI-420 material, after heat-treatment and machining process as engineering drawing, there was found defect (crack) in all areas of holes at flange such as in drawing bellow :
The crack are appears when these part was sent to customer after at about 24 hours, it indication as Delay Cracking.
3. Step of process production :
Raw Material
Roughness Machining
Heat-Treatment
NDT and Hardness Inspection
Final Machining
Final Inspection (NDT & Hardness)
Delevery
Rejected by Customer
4. Heat-Treatment
Heat-treatment was conduct as below, these process to achieve of Hardness 40 – 50 HRC
4.1. Hardening.
1 Hr
1 Hr
1 Hr
Oil Quench
Time
Temp (oC)
1000
760
540
4.2. Tempering
Temp (oC)
1 Hr
Time
540
5. Step of Analysis
Here are step of analysis as below :
Part Reject
DATA :
- Chemical Composition
- Micro Structure
- Hardness
Analysis and Conclusion
6. Data
6.1. Chemical composition was conduct by “Atomic Emission Spectrometer / Spark” method.
Element
Content (%)
C
0.75672
Si
0.22644
Mn
0.81994
P
0.02824
S
0.007
Ni
0.54478
Cr
15.40220
Mo
0.95183
V
0.10340
Cu
0.06945
6.2. Micro structure and Hardness was conduct as (see attachment)
- The material before Heat-treatment are about 25 HRC
- Hardness after Heat-treatment are 60 HRC
7. Analysis and Evaluation
7.1. Chemical Composition.
From data laboratory that materials was made from AISI-420 or Martensitic Stainless steel, but there is Carbon content are to High (=0.75 %C), the martensitic stainless steel AISI 420 commonly consist of , carbon are 0.15 to 0.20, as indicated to code 420, that mean :
4 XX are series of Martensitic
X 20 are Carbon content 0.15 to 0.20 %
7.2. Microstructure
- After Heat-treatment the micro structure are consist of Ferit in a matrix Martensite tempered and carbide.
- There is some crack was found in areas of ferrite phase.
7.3. Hardness are 60 HRC
7.4. Dimension
- In these part are different thickness (thick wall) in hole area
- In the area of holes there are not Radius.
Conclusion :
- The carbon content is too high so there caused form of Martensit and the excessive carbide will formed in the grain, as look like in the fig.1.
- From the micro structure as indicated in fig 1, there are ferrite phase in a matrix Matensite and carbides disperse between it. The martensite and carbide are very hard but Ferrite is soft and weak soo the presence of ferrite is initiate a Crack.
- In the Quenching process the martensit has internal stress and Tempering is mean to release it but if the temperature and holding time is too high, the structure be comes ……
- The achieved of hardness is too high (60 HRC) for this part (with the design of part), it may be enough 50 HRC.
- The difference Thickness of the hole areas and
