Materials Selection Analysis for a Hinge Pin on a Diving Bell Door

Introduction

A diving bell has two doors, manufactured from AISI 316 pressure grade stainless steel, which open and close by sliding against a hinge pin. The hinge pin (AISI 630 in the H1035 condition) was chosen for its galling resistance. Over time the hinge pin showed severe galling, resulting in difficulty opening and closing the door. Severe adhesive wear was found on both door and pin (Figure 1). The same phenomenon occurred when a copper beryllium alloy was used.

1

Figure 1: Severe galling on AISI 630 pin

Investigation

The hinge assembly is an H7G6 sliding fit, recommended for slow sliding mechanisms.  In both cases, the hinge was machined down to a surface finish of 1.6µm.  No lubricant was applied to the hinge assembly.  The assembly is immersed in salt water. Therefore the material must be resistant to corrosion in stagnant salt water.  The strength requirements of the hinge pin did not have to match the specifications for the AISI 630 H1025 stainless steel and the C 172000 copper beryllium alloy

What We Found

Results

The damage is indicative of galling, which occurs when two sliding surfaces are in contact with one another and have time to cold weld. On sliding, surfaces are broken and galling occurs. This is more likely when two like surfaces are in contact. Cold welding can be prevented by machining to appropriate surface finish, application of lubricant and correct material selection. Material selection criteria are shown in Table 1.

Table 1 – Materials Selection Criteria

Requirement Reason
Material must be resistant to corrosion in salt water The hinge pin assembly is immersed in salt water.  Further, a crevice will exist between the pin and the door.
Material must be resistant to cold welding and consequent galling. Galling is the result of cold welding between mating surfaces.  The hinge pin remains locked in position for periods of time.
The material must have sliding and adhesive wear resistance The hinge must be able to operate freely.

 

Various materials were investigated based on the criteria shown above.  Surface finish and lubrication requirements for such an assembly and fit were investigated.

 

The hinge pin has a surface finish of 1.6µm, which is greater than the specified range of 0.25 – 1.5µm for stainless steels in sliding wear contact.  Therefore if stainless steel is used, a finer surface finish should be investigated.  Table 2 shows the stainless steels that were investigated.

Table 2 – Stainless steels investigated for the hinge pin application

Material Reason for selection Comment
AISI 630 H1025 Previously selected Good hardness; good salt water performance; used for shafts and applications where crevice corrosion can occur.
AISI 630 Annealed May be easier to machine to 0.5µm surface finish Same composition as AISI 630 H1025, but is softer and easier to achieve a better surface finish, which may reduce galling.
AISI 630 H900 Harder than AISI H1025; can be heat treated post machining Same corrosion performance; if it can be machined to 0.5µm and heat treated to a greater hardness could provide excellent resistant to galling.
AISI 440 Hardness of 56HRC Very high hardness; difficult to machine; does not have the same corrosion performance; may not perform well in salt water
SAF 2205 Dual phase austenite ferrite structure which may be better for galling resistance Performs well in salt water; good hardness; machined relatively easily.  If can optimise surface roughness post machining, may show very good galling resistance.

 

The SAF 2205 and the AISI 630 are the most suitable stainless steels due to the corrosion resistance. Various copper alloys were considered for the application (Table 3).

Table 3 – Copper alloys investigated for the hinge pin application

Material Reason for investigation Comment
C 613000(aluminium bronze) Good corrosion resistance; recommended for sliding wear Lower strength than AISI 630 and C 172000 used previously
C 624000(aluminium bronze) Good corrosion resistance; recommended for sliding wear Lower strength than AISI 630 and C 172000 used previously
C 172000(Copper Beryllium) Previously used for hinge pin High strength; little reference to it as a bearing material

 

Table 4 shows the material properties for the materials investigated.

Table 4 – Material Properties

Material

Hardness

Yield Strength

MPa

UTS

MPa

Elongation to Failure %

Charpy Impact Strength J

AISI 630 Annealed

36HRC

AISI 630 H1025

38HRC

1158

1117

16

54

AISI 630 H900

44HRC

1365

1262

15

21

AISI 440

58HRC

1280

1750

4

19

SAF 2205

270HB

440

660

25

C 175000AT

36-42HRC

1000

1378

4-10

C 175000 TH04

95-102HB

690-825

760-895

8-15

240

C 613000

82HRB

240

540

35

81-88

C 624000

92HRB

330

655

14

15

 

If toughness and resistance to shock loading and fast catastrophic failure is necessary, then a material with a high charpy impact energy and high elongation to failure should be considered. The aluminium bronze C 613000 has good toughness and is a better option than C 614000, which does not show much improvement in strength for reduced toughness. SAF 2205 had good elongation to failure and is likely to show as good as, or better, toughness than AISI 630 in the H1025 condition. The AISI 630 in the H900 condition has a marked improvement in strength and hardness without large loss in ductility and hence is a better option than the AISI 630 H1025.

Conclusion

  • To reduce galling, the surface finish, lubricant selection and material selection must be optimised.
  • It is recommended that surface finishes of 0.5-1.0µm be tested for stainless steel. For the copper alloys, a surface finish of 0.5µm is probably optimal.
  • A lubricant that contains a solid should be selected. This creates a barrier between surfaces, preventing cold welding.
  • Of the stainless steels, SAF 2205 and AISI 630 in the H900 condition showed the best fitness for purpose.
  • Of the copper alloys, the aluminium bronzes showed the best fitness for purpose, specifically the C 613000 alloy.

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