Preliminary Evaluation of Maraging Steels on Its Application to Full Ocean Depth Manned Cabin

WANG Fang,HU Yong,CUI Wei-cheng

(Shanghai Engineering Research Center of Hadal Scicence and Technology,College of Marine Sciences,Shanghai Ocean University,Shanghai 201306,China)

Eqs.(4)and(5)can be combined to calculate the fatigue life under zero stress ratio.Six parameters need to be determined.If mean stress effect is considered,the Smith-Watson,and Topper(SWT)relationship can be adopted,

Preliminary Evaluation of Maraging Steels on Its Application to Full Ocean Depth Manned Cabin

WANG Fang,HU Yong,CUI Wei-cheng

(Shanghai Engineering Research Center of Hadal Scicence and Technology,College of Marine Sciences,Shanghai Ocean University,Shanghai 201306,China)

Full ocean depth manned submersible is to be designed for further exploring the deep sea trenches.When it is for a certain depth,the successful development of such an efficient system depends on the availability of a suitable material for the manned cabin,which contributes significantly to the weight of the manned submersible.The material must be available for construction,and should have the capability to withstand a very high external pressure and have other suitable properties to withstand hazardous environmental conditions.For the design of the full ocean depth manned cabin, the maraging steel has the greatest attraction due to its unique properties,such as high strength with excellent fracture toughness,ease of machining and above all distortion free thermal processing.The 18 percent nickel-cobalt-molybdenum family of maraging steels is thought that they may be a good candidate for full ocean depth manned cabin,so reasonable evaluation on their comprehensive properties should be made for completing the design of the manned cabin.In this paper,the material selection methods and some essential properties to be investigated in experiments will be discussed, which will provide initial guidance for subsequent experimental study and preliminary design.

maraging steel;material selection;fatigue;damage tolerance;crack growth

0 Introduction

Designers of submersibles have continually strived to reduce the structural weight,thereby increasing the achievable range/speed/payload.For underwater vehicles,W/D(weight-todisplacement ratio which is used to evaluate the structural efficiency of underwater vehicles) of the pressure hull may be about 20%of that of the vehicle[1].Therefore,the weight of the pressure hull of an underwater vehicle can potentially reduce the total weight of the underwater vehicles[2].When the submersible is to be designed for a certain depth,the successful development of such an efficient system depends on the availability of a suitable material whichmust be available for construction,and should not only have the capability to withstand a very high external pressure but also have other suitable properties to withstand hazardous environmental conditions.

There is a number of material types commonly employed for pressure hull of underwater vehicle applications.From previous work on submersibles,it is already known that advanced materials with diverse properties will be required.Better performances should be considered in selection of the material,which include better strength-to-weight ratio,good resistance to corrosion,better ductility,low weight capacity,capability to withstand high external pressure,better fatigue and creep property,material availability and cost,fabrication properties(better weldability,casting or forging properties),tolerance to temperature,good sound absorption qualities,etc.Materials of possible interest for pressure-hull construction are high strength steels, light alloys including high-strength aluminium and titanium,FRPs,metal-matrix composites (MMC),ceramic composites,acrylics and high strength glass[3].Some of the currently used materials for underwater pressure hulls and their main properties can be found in Ref.[4].

In particular,the manned cabin of deep-sea submersibles is the most representative underwater pressure hull.This technical development greatly expanded the operating range and improved the operational efficiency of the human occupied vehicles(HOV)used in scientific research.The appropriate material selection will guarantee the efficiency and safety of the manned cabin,which is the most critical component and bears most of the weight for the HOV and will be specially focused on in this chapter.Qu(2013)lists the properties of candidate materials for manned cabin[5].

For the design of the full ocean depth manned cabin,the maraging steel has the greatest attraction due to its extreme high strength.Maraging steels exhibit unique properties,such as high strength with excellent fracture toughness,ease of machining and above all distortion free thermal processing.The development of the nickel maraging steels began in the Inco research laboratories in the late 1950s and was based on the concept of using substitutional elements to produce age-hardening in the low-carbon iron-nickel martensitic matrix.Hence the term‘maraging’was given to them to signify this strengthening mechanism.The work led to the discovery that balanced additions of cobalt and molybdenum to iron-nickel martensite gave a combined age-hardening effect appreciably greater than the additive effects of these elements used separately.Furthermore,the iron-nickel-cobalt-molybdenum matrix was found to be amenable to supplemental age-hardening by small additions of titanium and aluminium.Thus the 18 percent nickel-cobalt-molybdenum family of maraging steels was developed.

Even though marging steels are acknowledged that they may be reliable for full ocean depth manned cabin,reasonable evaluation on their comprehensive properties should be made for completing argumentation on different international resources of material,which is being conducted in Hadal Science and Technology Research Center of Shanghai Ocean University in China.However,useful public reports on comprehensive properties of maraing steels are difficult to be found.Then validation experiments will be arranged for material selection and de-sign purpose.In this paper,the material selection methods and some essential properties to be investigated in experiments will be discussed,which will provide initial guidance for subsequent experimental study and preliminary design.

1 Material selection methods

The current available design rules of submersible structures from ship classification societies(specific rules for submersibles are included)are based on the philosophy of strength design,namely,the ultimate strength of the hull should be higher than the specified collapse pressure[6].And the material selection method is based on the strength design principle.A spherical manned cabin is usually applied for deep-sea manned submersibles just like‘Jiaolong’of China has the spherical manned cabin made of high strength titanium alloy.When the full ocean depth manned submersible is to be developed,the same material as‘Jiaolong’is considered,however,if a similar sphere is manufactured using the titanium alloy,the manned cabin thickness will exceed 110 mm,which could not be manufactured with the current manufacture ability of most countries considering the thin-off amount and size of the sphere.Due to constraint of strength of titanium alloy,if full-ocean-depth submersible is designed to have enough volume to take 3 occupants,a novel manned cabin design of double intersecting spheres has ever proposed[7]and optimized.However,the connecting part of the two spheres is difficult to construct for meeting all practical and theoretical requirements.Then the idea needs more investigation.Other materials such as maraging steels step into full ocean depth manned cabin design process.The characteristics of maraging steels have been simply introduced in Chapter 0,while the candidate materials for full ocean depth manned cabin are limited to 18Ni grade after a preliminary investigation and research in terms of their application history,performance and current manufacturing capability.

There are basically four wrought commercial maraging steels of the 18 percent nickel family and one cast grade currently available from special manufacturers.The nominal compositions and 0.2%proof stress values are presented in Tab.1[8].

Tab.1 The 18 percent nickel maraging steel[8]

In the development of MIR submersibles in 1980s,at that time,Rauma Repola developed new techniques to produce high strength,high nickel-content steel for pressure hulls.This‘Maraging Steel’can be traced back to Lockheed’s design of the DEEPQUEST,which used this alloy for its two pressure spheres.However,the welding between the two spheres proves to bea significant problem,and the decision is made to avoid any welding on the MIR hulls.Two casted half-spheres undergo the necessary machine tooling and then are put together and fastened with bolts.The sphere has three viewports:one central with the inner diameter of 200 mm and one on each side with a 120-mminner diameter.These viewports provide the crew with an adequate field of vision during their work under the surface[9].

At the same time,higher requirements have been made for improved safe and economic design and operation of the submersibles,and the material selection criterion has paid more attention to the synthetic properties of materials of interest.The pressure hull of deep manned submersibles during their service life will experience periods of both fluctuating and steady stress.Then fatigue property of the structure has received attention.It is believed that the increased fatigue failure can be attributed to higher applied stresses,higher yield strength of the material,the existence of an inhomogeneous microstructure,etc.Basic fatigue property of the material can be expressed by S-N curve and the fatigue limit at a prescribed stress cycles can be taken as one of the material selection guideline.With the advance of structure integration and damage-tolerant design concept,the fracture toughness and crack growth rate may be also involved in the synthetical material selection criterion and optimized design principle. Fracture toughness and fatigue crack growth rate are affected largely by the microstructure, the forging processing,and the heat treatment processing.Inside defects of the material are ineluctable.The essence of damage tolerant design concept is to consider the stable crack growth properties and residual strength of the material with defects.Therefore,the prospective material selection criterion should include the static strength,fatigue strength and damage tolerant properties of the candidate material.They are dependent on one another and their relations should be taken into account.For example,it has been widely recognized that the fracture toughness of many metal structures will decrease with the increase of material tensile strength[10-11]. Considering the conflict between static strength and fracture toughness,some material selection criteria were established in aeronautical field while not available in submersible technology. A unified principle with multi material properties and design requirements need to be proposed in the future to guide the new material research and development.

The authors have made some attempts in this aspect during the development of 4 500 m-class deep manned submersible in China.At that time,several damage tolerance types of titanium alloys have been developed for this purpose.For the material selection and design of the 4 500 m depth manned submersible,systematic study on material properties were carried out by means of theoretical analysis,numerical simulation and precise small specimen tests. And their fracture and fatigue crack propagation rate properties were discussed in detail.Fatigue and fracture tests are conducted on the two types of candidate titanium alloys and these results were presented and discussed by normalization considering the strength of the material from the viewpoint of macro properties while the microstructure effect has not been touched upon,although beneficial microstructure effect is the intrinsic factor to improve damage tolerant properties of the material.The methodology can provide basis for further evaluation of ma-terial experiment results on maraging steels.

Certainly,together with the development of submersibles,related regulations have been established,no matter they are reliable enough or not.Among some generally accepted guidance rules,Russian Maritime Register of Shipping specified the regulation of‘Steel for Hull Structures’,which is used for steel intended for construction of pressure hulls of the manned submersibles and ship’s diving systems.Steel for pressure hulls of the submersibles and diving systems shall meet the following additional requirements,which shall be confirmed during its approval:

(1)Reduction of area of steel with yield stress Reh>490 MPa shall be not less than 50 per cent;(where Rehis the same as σsin the present paper.)

(2)Impact energy KVLon longitudinal specimens depending on the purpose of the structure,use of heat treatment after welding,steel strength level and thickness of structural members shall be not less than that given in Tab.2;in this case,impact testing temperature Ttis determined from the formula Tt=Td-20°C.

Tab.2 Impact energy required for steels

(3)Anisotropy of the steel plate properties in longitudinal and transverse directions is defined by impact testing,the ratio KVT/KVLin this case shall be not less than 0.8;

(4)For the steel plate of 40 mm thick and more,impact tests are carried out on test specimens cut out from the middle of the plate thickness;in this case,impact energy KV shall be not less than that required in Part XIII‘Materials’of the Rules for the Classification and Construction of Sea-Going Ships and specified in Tab.2 of this paper;

(5)For the steel plates,static fracture tests are carried out on notched specimen of the full-scale thickness with determination of the amount of fiber component in the fracture, which shall be not less than 70 per cent.The tests are carried out according to the procedure approved by the Register;

(6)Properties of the material used in the structure(after bending,die forcing,etc.)shall be not lower than those required in this Chapter.Among the controlled characteristic are ReH(at compression and tension),Rm,A5,Z and KV.

2 Damage tolerance performances of 18Ni grade maraging steels

Yield ratio is one of the simplified indicators to show the plastic deformation and strainhardening capacity of the material. Based on the tensile properties data of the existing 18Ni grade maraging steel,Fig.1 is made to show the value of yield ratio versus yield strength for 18Ni grade maraging steel.It can be seen that the yield ratio σs/σbgoes up with the increase of yield strength.

Plane strain fracture toughness is an important indicator to represent the crack resistance property of the material.Fig.2 and Fig.3 show respectively the value of plane strain fracture toughness versus yield strength and tensile strength for 18Ni grade Maraging steel.The experimental data are from Fan and Dai(1995)[12]and Gangloff(2003)[13]respectively.The plane fracture toughness KICwill decrease rapidly with the increase of strength level.Wherein 18Ni(250)with yield strength of 1 700 MPa has the plane strain fracture toughness level of 85-110 MPa,but the value of 18Ni(350) with yield strength of 2 400 MPa rapidly reduces to 30-50 MPa.

Fig.1 The value of yield ratio versus yield strength for 18Ni Maraging steel

Fig.2 The value of plane strain fracture toughness versus yield strength for 18Ni Maraging steel

Fig.3 The value of plane strain fracture toughness versus tensile strength for 18Ni Maraging steel

As the experience of human being to manufacture deep-sea manned cabins is limited,there is still no such a formal material selection criterion as that for aircrafts.When the full ocean depth manned cabin is being developed,there will be some special considerations on proper material selection.However,purely from the point of view of the comprehensive performance evaluation of materials,the material selection criteria in the field aviation can be taken as references.Those criteria for aircrafts were developed with the concept of structural integrity and damage tolerance,and accordingly the material selection criterion just based on the strength of intact structure was certainly changed.The new criterion widely used in aircraft design nowadays as a formal material selection standard put emphasis on the residual strength and fatigue property of damaged structure(or aged structure),together with the traditional elemental issues just considering intact structure(or new manufactured structure).As has been described in a lot of literature,when static strength is the only criterion for material selection,some high strength metal materials have absolute privilege.However,lots of failure accidents of aircrafts prove the misleading of the criterion,because high strength metal materials are more sensitive to defects like cracks while materials with defects should be evaluated using fracture mechanics based method,namely,fracture toughness based criterion.From Figs.2-3,it can be seen that the fracture toughness of 18Ni maraging steel decreases with increase of its strength level.The contradiction is the main reason for establishing a reasonable criterion comprehensively considering the static strength and fracture toughness.More specific illustrations on this issue can be found in Cao(2002)[14].In the present paper,three typical criteria set up for different types of aircrafts are listed below for reference.Therein,Eq.(1)and Eq.(2)result in similar tendency as drawn in Fig.4.

Rolfe(1970)[15]proposed a criterion using the relationship of KICand σy,and considering the effect of component thickness,t.,as follows,

If the criteria expressed in Eqs.(1)and(2)can be applicable for 18Ni maraging steels, their damage tolerance properties can be evaluated in Fig.4.The material data are fitted from those in Fig.3.The other two curves are plotted based on Eqs.(1)and(2).The two criteria are basically in the same level.The intersection of the material data curve and criterion curve is located near the point of σb=1 900 MPa,which means that only 18Ni(200)and 18Ni(250)are in accordance with the requirements for damage tolerance material.And for the candidate material based on strength design criterion for deep-sea manned cabin,18Ni(250),with yieldstrength of 1 700 MPa,its fracture toughness should be approximately over 80 MPam0.5.However,with the rapid development of material science and metallurgy,18Ni(300)and even 18Ni(350)are highly improved to have better damage tolerance performance.

These guidelines,of course, are based on their application requirements and experience in aviation.However,they provided a way of thinking for establishment of the material selection criterion for deep-sea manned cabin when loading and stress conditions are sufficiently considered. Based on the analysis in this Chapter,18Ni(250)will be separately analyzed in the following Chapters.

Fig.4 Fracture toughness versus ultimate tensile strength of 18Ni maraging steels

3 Durability of 18Ni(250)

3.1 Low-cycle fatigue properties of 18Ni(250)without dwell effect

Fatigue life assessment must be conducted before evident defects are detected,typically for the new spherical hull.The assessment method is generally based on low-cycle fatigue properties obtained from costly experiments or simply from some prediction methods.The latter are usually expressed by explicit formulas with parameters estimated from tensile tests,which can be conveniently applied in large fatigue assessment programs.From 1960s,several integrated prediction methods have been proposed,such as four-point correlation method[16],modified four-point correlation method[17],universal slope rate method[16],modified universal slope rate method[18],hardness method[19].Estimation methods as introduced above are valuable when test data are limited in the material selection period of a component.Review and evaluation on those estimation methods can be found in Zhang et al(2011)[20].Four-point correlation method which is obtained by experiments of 29 types of metal materials is adopted here comparing to the test data from literature.

In a tentative inspection procedure,the application of effective explicit equations will largely simplify the calculation process.In this section,the four-point correlation method will be validated through test data and will be used for evaluating the low-cycle fatigue life of maraging steel for deep-sea manned cabin.

As is known,the cyclic stress-strain curve can be commonly expressed by Ramberg-Osgood form,

And the Coffin-Manson relationship is used to expressed the relationship between the total strain amplitude and the life,

Eqs.(4)and(5)can be combined to calculate the fatigue life under zero stress ratio.Six parameters need to be determined.If mean stress effect is considered,the Smith-Watson,and Topper(SWT)relationship can be adopted,

Compared Eqs.(5)and(6),the constants for the strain life curve can be related to those for the cyclic stress-strain curve,

Thus,of the six constants H′,n′,σf′,b,εf′and c,only four are independent.

The four-point correlation method can be expressed by following series of equations[13],

The above equations are used to predict the low-cycle fatigue properties of 18Ni (250),as shown in Fig.5,together with the data plotted based on the parameters provided in Banas et al(1988)[21].And Tab.3 lists the monotonic and cyclic properties of 18Ni(250)in Banas et al(1988)[21].The two curves agree well showing the reasonability of the four-point correlation method for 18Ni(250),which will provide prediction method when test data is limited in the preliminary design with 18Ni maraging steels.

Fig.5 The test results(Banas et al,1988)[21]compared with prediction curve by the four-point correlation method(Manson,1965)[16]

Tab.3 Monotonic and cyclic properties of 18Ni(250)[21]

3.2 Fatigue properties of 18Ni(250)with dwell effect

As the manned sphere during its service life will experience periods of both fluctuating and steady stresses with creep and fatigue involved,which is a rather complex variable amplitude pattern and quite different from the common cyclic fatigue loading,as shown in Fig.6.It is also the difference between laboratory studies and actual situation.However,research in this area has not been carried out into an in-depth phase.In this section,more is to emphasize the importance of the issue and assumes a tendency for dwell fatigue life of 18Ni(250)based on a prediction model proposed by the authors recently[22].

Fig.6 The comparison of the dwell fatigue loading with fatigue loading

Based on the judgment on the dwell effect issue,the authors have put forward the following formula for stress versus cycles to failure,which can be expressed by a segmented function[22],

where σmaxis the maximum stress in each cycle;εais strain amplitude;E is elastic modulus of the material;The first part when σmax≤σbin Eq.(2)is the expression of SWT model for strain based approach to fatigue(Smith,Waton and Topper,1970),where σf′,εf′,b and c are tabulated as the material properties,which can be determined by experiments and some experimental results have been listed in Tab.2.

An illustration of three parts of the peak stress versus cycles to failure is depicted in Fig.7. Based on the model,parts of S-N curve of 18Ni(250)under fatigue and dwell fatigue loadingcan be approximately drawn in Fig.8. These curves only show the author’s judgment on the trend of changing tendency.Model validation based on experiments for 18Ni(250)will be carried out in subsequent investigations.

3.3 Crack growth rate property of 18Ni(250)without dwell effect

The crack growth characteristics of maraging steel have not reported enough in recent years while some results can be found in literature published before 1980s when the application of maraging steel is a hot issue.In 1989,Diwakar et al studied the cyclic crack growth rate applying in sinusoidal and block loading conditions on compact tension specimen and surface crack tension specimen[23].They gave da/dN against K plots for parent metal and weldments of 18Ni(250)which shows the yielded Paris Law(logda/dN= logC+nlogΔK)constants C and n as 1. 05×10-10and 2.2 for parent metal,and 0.27×10-10and 2.27 for weldments respectively.And the threshold stress intensity in the weldment is 0.6 that for parent metal,however,the details of threshold stress intensity range has not been written in text.In order to better understand the crack growth properties of 18Ni(250),the authors carried out a series of crack growth experiments on CT specimens in four load ratios of 0.1,0.3,0.5 and 0.7. Fig.9 shows the experimental results. A relatively big value of stress intensity factor range in unstable crack growth region can be observed.

Fig.7 An illustration of three parts of the peak stress versus cycles to failure

Fig.8 Approximately estimated parts of S-N curve of 18Ni (250)under fatigue and dwell fatigue loading

Fig.9 da/dN against ΔK plots for parent metalof 18Ni(250) in different load ratios

3.4 Crack growth rate property of 18Ni(250)with dwell effect

At the same time,Diwakar et al(1989)[23]studied the crack growth characteristics of 18Ni (250)under block loading cycles which are similar to the load pattern of manned sphere except the cycle frequency(load pattern of manned sphere has much longer loading time,dwell time and unloading time),but the results can illustrate some tendency of dwell effect on crack growth. Fig.10 shows the schematic diagram of blocking loading.The loading pattern includes the proof pressure test cycle and static test cycle.Interesting observations have been found which are different for high level and low level of proof pressure cycle respectively.High level of proof pressure will induce a sudden initial high crack growth rate and its plastic zone retards further crack growth which is seen as very small or negligible da/dN during subsequent block loading cycles.On the other hand,if the initial proof pressure is low,the crack growth will increase with the increasing number of block loading.And a definite conclusion can be drawn that the da/dN for blocking loading(cyclic load with dwell time)is an order of magnitude higher than that for sinusoidal loading(cyclic load without dwell time). This is also regarded as the most important observation for the data on da/dN between the laboratory and service condition,indicated by Diwakar et al(1989)[23]. The introduction on above results can be an example of dwell time effect for 18Ni (250),which proves the importance of the issue even for the problem of manned cabin touched upon in the present paper. To illustrate it more clearly,a comparison of limited data given by Diwakar et al(1989)[23]is re-described by the present authors in Fig.11. Further experimental studies should be made for the crack growth characteristics of 18Ni(250)under dwell fatigue loading condition when validating its applicability for manned cabin of submersibles.

Fig.10 The schematic diagram of block loading by Diwakar et al(1989)[23]

Fig.11 Comparison of crack growth data under two loading conditions given by Diwakar et al(1989)[23]

To obtain the full view of the crack growth characteristic of 18Ni (250)under dwell fatigue loading, proper experiments should be con-ducted.Overall test procedure will be carried out in the future’s investigation.In the current preliminary material validation period,some estimation methods can be adopted for it as similar to the way in Section 3.1.For crack growth curve under fatigue loading,three regions are included which are threshold region,stable growth region and unstable growth region.Then two critical values for the transition of the three regions shall be determined,namely,threshold stress intensity factor range and fracture toughness.In Fig.3,the plane strain fracture toughness of 18Ni(250)has been indicated and can be the transition value between stable and unstable crack growth regions.As for the threshold stress intensity factor range,the authors have not found related data yet.Then the following estimation equations will be used for the preliminary design,which can be further validated in the future’s work.

Vosikovski(1979)[24]proposed an empirical formula for steels,

And,Farahmand and Nikbin(2008)pointed out that the value of△Kthfor a wide range of alloys was found to be propotional to the final elongation,εf,and established an empirical relationship at zero load ratio as follows[25],

Based on above two equations and the properties listed in Tab.3,the value of△Kthis 3.58 or 1.880 7 MPa m1/2at zero load ratio.A mean value of 2.73 MPa m1/2is recommended presently.Then the crack growth curve of 18Ni(250)under fatigue loading can be simply drawn,as shown by the solid line in Fig.12.

Under the dwell fatigue loading, two assumptions will be made,(1)The threshold stress intensity factor range will be the same as that under fatigue loading;(2)The unstable stress intensity factor range will be lower than the plane strain fracture toughness. The first assumption is made based on former studies[26]showing that the S-N curve will overlap for fatigue and dwell fatigue loading when stress level is low enough,which means that the fatigue limit should be the same value while the threshold stress intensity factor range is related to fatigue limit.From another point of view,dwell time effect is in association with the room temperature creep property of the material,while creep will not occur under low stress condition.The second assumption is made based on the difference between the linear and nonlinear values of stress intensity factor range[27].When dwell time exists in each load cycle,thechange of the plastic zone due to dwell time above the normal cyclic plastic zone should be considered when stress intensity level is large enough approaching to unstable growth period, which directly extends the modified crack length during the calculation of nonlinear stress intensity factor and then causes the acceleration effect on crack growth.Then unstable growth condition should be the modified one with the linear part included while the abscissa in Fig.12 is just the linear part of stress intensity factor range.So the unstable growth period for dwell fatigue loading shall be in advance comparing to that for fatigue loading,as shown by the dashed line in Fig.12.

Fig.12 Estimated crack growth curve under two loading conditions

4 Summary and conclusions

The material selection methods and some essential properties to be investigated in experiments have been discussed in this paper.Based on that,summary and conclusions can be made as follows.

(1)The current material selection method for manned cabin is based on the strength design principle,while with the improved requirements on safety,current design principle must be combined with comprehensive understanding on materials,especially durability.Existing material selection criteria are summarized and discussed in the paper;

(2)The candidate material for full ocean depth manned cabin is limited to 18Ni grade maraging steels after a preliminary investigation and research in terms of their application history,performance and current manufacturing capability.The damage tolerance related performances of 18Ni grade maraging steels including yield ratio,fracture toughness and their tendencies to strength are evaluated.Some guidelines based on their application requirements and experience in aviation are as references to provide a way of thinking for establishment of the material selection criterion for deep-sea manned cabin when loading and stress conditions are sufficiently considered.Based on those,18Ni(250)will be the most promising material for full ocean depth manned cabin;

(3)The durability of 18Ni(250)is evaluated,however,it aims at indicating some simplified assumptions used for preliminary design at which stage experimental data are limited. Low-cycle fatigue properties and crack growth properties are separately estimated using existing theories,including properties under dwell fatigue loading.It is emphasized that dwell fatigue properties should be sufficiently considered as there may be a significant difference comparing fatigue and dwell fatigue problem.Subsequent experimental investigation will validate the theoretical assumption and simplified methods in the future’s work.

Acknowledgments

This work is supported by the State Key Program of National Natural Science of China‘Structural Reliability Analysis on the Spherical Hull of Deepsea Manned Submersibles’(Grant No.51439004),the General Program of National Natural Science Foundation of China‘Studyon the Design and Life Calculation Method for the Maraging Steel Sphere of Full-ocean-depth Manned Submersible’(Grant No.51679133)and the scientific innovation program project‘Key technology research and experimental validation of deep manned submersible’by the Shanghai Committee of Science and Technology(Grant Nos.14DZ1205501&14DZ2250900 &15DZ1207000).

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马氏体镍钢用于制造全海深载人舱的可行性初探

王芳，胡勇，崔维成

（上海海洋大学海洋科学学院，上海深渊科学工程技术研究中心，上海201306）

为进一步探索深渊海沟，需要研制全海深载人潜水器。在给定设计深度下，潜水器的成功研制取决于对载人舱选材的合理性。合理选材使得载人舱的重量较小，同时，材料具备良好的综合性能使其能够承受非常高的外部压力及适应严峻的环境条件。在全海深载人舱设计过程中，马氏体时效钢以其较高的强度、良好的韧性、较好的可加工性能及热处理性能而备受关注。其中，18Ni系马氏体时效钢因具备较好的综合性能而作为全海深载人舱的备选材料，因此在载人舱设计之前需要对备选材料的综合性能进行合理的评估。该文对载人舱材料选择标准进行了探讨，以此为基础，针对18Ni系马氏体镍钢的重要性能指标进行了分析，分析结果将为后续的材料试验研究和载人舱初步设计提供理论指导。

马氏体时效钢；选材；疲劳；损伤容限；裂纹扩展

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王芳（1979-），女，上海海洋大学副研究员；胡勇（1975-），男，上海海洋大学研究员；崔维成（1963-），男，上海海洋大学教授，博士生导师。

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10.3969/j.issn.1007-7294.2016.12.006

1007-7294（2016）12-1557-16

Received date：2016-09-29

Foundation item:Supported by the State Key Program of National Natural Science of China(Project No.51439004); The General Program of National Natural Science Foundation of China(Grant No.51679133); The Scientific Innovation Program Project by the Shanghai Committee of Science and Technology (Project Nos.14DZ1205501&14DZ2250900&15DZ1207000)

Biography：WANG Fang(1979-),female,associate professor,E-mail:wangfang@shou.edu.cn; HU Yong(1975-),male,professor;CUI Wei-cheng(1963-),male,professor/tutor.