This specification deals with the standard requirements for superstrength alloy steel forgings for highly stressed structural members. Covered here are eight types (twenty-seven grades) of alloy steels, namely: quenched and tempered steels (Grades 11, 12, 12a, 13, 21, 22, 23, 31, 32, and 33); air hardened steel (Grade 41); martensitic stainless steels (Grades 51, 52, and 53); No. 1 precipitation hardened stainless steel (Grade 61); No. 2 precipitation hardened stainless steel (Grade 64); No. 3 precipitation hardened stainless steels (Grades 62 and 63); maraging steels (Grades 71, 72, 73, 74, and 75); and miscellaneous steels (Grades 81, 82, 83, and 84). The steels shall be manufactured by melting process and shall be finished by hot-worked forging. Forgings may be furnished in one of the following conditions: stress relieving; annealing; solution treatment; solution treatment and ageing; normalizing; normalizing and tempering; or quenching and tempering. Heat and product analyses shall be employed to assess the conformance of steels with required chemical compositions. Forgings shall examined by tension and Charpy impact tests and conform to required mechanical properties such as yield strength, tensile strength, elongation, reduction of area, minimum room-temperature Charpy V-notch energy absorption, maximum annealed hardness, and section size capability. Nondestructive tests, such as magnetic particle, liquid penetrant, and ultrasonic inspection procedures, shall also be performed.1.1 This specification covers requirements for forged steel shapes for highly stressed structural members requiring yield strengths in excess of 140 ksi [965 MPa].1.2 This specification is not intended for applications limited by creep deformation.1.3 Twenty-eight grades are covered by this specification. Selection will depend upon design, service conditions, and mechanical properties required.1.4 Supplementary requirements are provided for use when additional testing or inspection is desired. These shall apply only when specified individually by the purchaser in the order.1.5 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.1.6 Unless the order specifies the applicable “M” specification designation, the material shall be furnished to the inch-pound units.1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 590元 加购物车

This specification deals with alloy structural steel plates. Plate specimens addressed under this specification are available in four treatment conditions (as rolled or hot rolled; annealed; normalized; and normalized and tempered), three structural forms (rectangular; circular and semi-circular; and sketch, including rings), five edge categories (mill edge; universal mill edge; sheared edge; gas cut edge; and special cut edge), and seven finishes (as rolled or hot rolled; blast cleaned; blast cleaned and oiled; pickled; pickled and oiled; painted with one prime coat; and painted with one prime coat and one finish coat). Steel materials shall be manufactured and furnished in defined procedures unless specified otherwise in the purchase order. Heat analysis shall be performed wherein steel specimens shall conform to required chemical compositions of carbon, manganese, phosphorus, sulfur, silicon, nickel, chromium, molybdenum, vanadium, and copper. Specimens shall also undergo tensile tests and shall conform to required values of tensile strength.1.1 This specification covers structural quality alloy steel plates.1.2 The plates are usually specified to chemical composition requirements, but tensile properties may also be specified.1.2.1 When tensile properties are required, the specific requirements must be compatible with the chemical composition, condition, and plate thickness.1.3 The plates are available in the following five conditions:1.3.1 Condition AR—As rolled (hot rolled),1.3.2 Condition A—Annealed,1.3.3 Condition N—Normalized,1.3.4 Condition NT—Normalized and tempered, and1.3.5 Condition QT—Quenched and tempered.1.4 The plates are available in the following three forms:1.4.1 Form I—Rectangular,1.4.2 Form II—Circular and semi-circular, and1.4.3 Form III—Sketch, including rings.1.5 The plates are available in the following five edge categories:1.5.1 Edge 1—Mill edge,1.5.2 Edge 2—Universal mill edge,1.5.3 Edge 3—Sheared edge,1.5.4 Edge 4—Gas cut edge, and1.5.5 Edge 5—Special cut edge.1.6 The plates are available in the following seven finishes:1.6.1 Finish 1—As rolled (hot rolled),1.6.2 Finish 2—Blast cleaned,1.6.3 Finish 3—Blast cleaned and oiled,1.6.4 Finish 4—Pickled,1.6.5 Finish 5—Pickled and oiled,1.6.6 Finish 6—Painted, one prime coat, and1.6.7 Finish 7—Painted, one prime coat and one finish coat.1.6.8 The plates are usually specified to have Finish 1.1.7 Supplementary requirements are provided for additional requirements that may be specified on the order.1.8 When the steel is to be welded, it is presupposed that a welding procedure suitable for the grade of steel and intended use or service will be utilized. See Appendix X3 of Specification A6/A6M for information on weldability.1.9 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.1.10 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 515元 加购物车

This specification covers standards for sizes of screened ferroalloys and alloy additives. Products shall conform to specified nominal size, standard ordered size, maximum allowable oversize, maximum allowable undersize, and friability rating. Description of the friability ratings of the ferroalloys is also provided.1.1 This specification covers standard nominal sizes and size tolerances of screened ferroalloy and alloy additive products. This specification provides a range of sizes as referenced in all ASTM specifications for ferroalloys and alloy additives.1.2 The sizes and tolerances allow for varying degrees of friability upon receipt of material since some attrition may be expected in transit, storage, and handling.1.3 Specifications of sieve sizes used to define tolerances are listed in Specification E11. Representative procedures for evaluation of each lot are described in Test Methods A610. Refer to Appendix X1 for applicable sieve designations (see Table X1.1).1.4 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the standard.1.4.1 This specification is expressed in both inch-pound units and in SI units (within the text, the SI units are shown in brackets); however, unless the purchase order or contract specifies the applicable M specification designation (SI units), the inch-pound units shall apply.1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 515元 加购物车

1.1 This specification covers cold-drawn cobalt alloy spring wire intended especially for the manufacture of springs.1.1.1 This specification requires capability properties after age hardening test samples of the cold worked spring wire to ensure final mechanical properties are satisfied. The spring is typically age hardened after fabrication to increase its strength. However, mechanical properties cannot be obtained from finished springs.1.2 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.1.3 Unless the order specifies an “M” designation, the material shall be furnished to inch-pound units.1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 515元 加购物车

8.1 For the purpose of determining compliance with the specified limit for requirements of the properties listed in the tables, an observed value or a calculated value shall be rounded as indicated in accordance with the rounding method of Practice E29.AbstractThis specification covers unalloyed and alloyed tantalum ingots prepared by vacuum-arc melting, electron-beam melting, or powder-metallurgy consolidation to produce consolidated metal for processing to various mill shapes. The materials covered by this specification are classified according to types: R05200; R05400; R05255; R05252; and R05240. The ingot metal for types R05200, R05255, R05252, and R05240 may be prepared by vacuum-arc melting, electron beam furnace melting, or a combination of these two methods. The metal for type R05400 is defined as powder-metallurgy consolidation unalloyed tantalum. The material shall conform to the chemical composition requirements for carbon, oxygen, nitrogen, hydrogen, niobium, iron, titanium, tungsten, molybdenum, silicon, nickel, and tantalum.1.1 This specification covers unalloyed and alloyed tantalum ingots prepared by vacuum-arc melting, electron-beam melting, or powder-metallurgy consolidation to produce consolidated metal for processing to various mill shapes.1.2 The materials covered by this specification are:1.2.1 R05200, unalloyed tantalum, electron-beam furnace or vacuum-arc melt, or both,1.2.2 R05400, unalloyed tantalum, powder-metallurgy consolidation,1.2.3 R05255, tantalum alloy, 90 % tantalum 10 % tungsten, electron-beam furnace or vacuum-arc melt, or both,1.2.4 R05252, tantalum alloy, 97.5 % tantalum 2.5 % tungsten, electron-beam furnace or vacuum-arc melt, or both, and,1.2.5 R05240 tantalum alloy, 60 % tantalum 40 % columbium, electron-beam furnace or vacuum-arc melt, or both.1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 515元 加购物车

This specification establishes requirements for gear bronze alloy castings with Copper Alloy No. C90800, C91700, C90700, C91600, and C92900. The castings may be furnished as one of three types: static chill, centrifugal chill, or sand cast. The alloys shall be subject to tensile strength, yield strength, elongation, and Brinell hardness tests. The Brinell readings shall be taken on the grip end of the tension test bar as described in the specification.1.1 This specification2 establishes requirements for alloys whose copper alloy numbers and compositions are shown in Table 1. The castings may be furnished as one of three types: static chill, centrifugal chill, or sand cast.1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 515元 加购物车

This specification covers three grades of zirconium and zirconium alloy forgings. The forgings shall be formed with conventional forging equipment normally found in primary ferrous and nonferrous metal plants. The forgings are furnished in three grades as Grade R60702, Grade R60702, and Grade R60705. Forgings shall be furnished in the annealed conditions. The material shall conform to the requirements as to chemical composition and tensile properties prescribed. Two tension tests shall be made from each lot. Two chemistry tests for hydrogen and nitrogen content shall be made from each lot of finished product. If the results of any tests of any lot do not conform to the requirements specified, retests shall be made on additional forgings of double the original number of the same lot, each of which shall conform to the requirements specified.1.1 This specification2 covers three grades of zirconium and zirconium alloy forgings (see 4.1).1.2 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.1.3 The following safety hazards caveat pertains only to the test method portion, Section 12, of this specification: This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 515元 加购物车

This specification covers an alloy containing palladium, silver, copper, gold, platinum, and zinc in the form of wire, rod, and strip for electrical contacts. The material shall be finished by such operations: cold working, heat treating, annealing, turning, grinding, and pickling. Material shall meet the requirements of chemical composition as specified. Knoop hardness test and tension test shall be made to conform to the specified requirements. Chemical analysis shall be performed by spectrochemical or wet analysis methods.1.1 This specification covers an alloy containing palladium, silver, copper, gold, platinum, and zinc in the form of wire, rod, and strip for electrical contacts.1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.1.3 It is the responsibility of the user to become familiar with all hazards including those identified in the appropriate Safety Data Sheet (SDS) for this product/material as provided by the manufacturer.1.4 The following safety hazard caveat pertains only to the test methods portion, Section 6 of this specification. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to become familiar with all hazards including those identified in the appropriate Safety Data Sheet (SDS) for this product/material as provided by the manufacturer, to establish appropriate safety, health, and environmental practices, and determine the applicability of regulatory limitations prior to use.1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 515元 加购物车

This specification covers a gold-rich, age-hardenable alloy in rod, wire, and strip form applicable to electrical contacts. The material shall be finished by cold working, annealing, turning, grinding, and age hardening. The materials shall conform to the specified chemical composition requirements. Tensile strength, elongation, and hardness shall be determined by tension and hardness tests. Chemical analysis shall be done by spectrochemical or wet analysis methods.1.1 This specification covers a gold-rich, age-hardenable alloy in rod, wire, and strip form applicable to electrical contacts.1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.1.3 The following precautionary statement pertains to the test method portion only, Section 7, of this specification. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to become familiar with all hazards including those identified in the appropriate Safety Data Sheet (SDS) for this product/material as provided by the manufacturer, to establish appropriate safety, health, and environmental practices, and determine the applicability of regulatory limitations prior to use.1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 515元 加购物车

This specification covers for titanium and titanium alloy wire. The chemical composition; physical properties such as density; and mechanical properties such as tensile strength, elongation and hardness are detailed.1.1 This specification covers titanium and titanium alloy wire as follows:1.1.1 Grade 1—UNS R50250. Unalloyed titanium,1.1.2 Grade 2—UNS R50400. Unalloyed titanium,1.1.2.1 Grade 2H—UNS R50400. Unalloyed titanium (Grade 2 with 58 ksi (400 MPa) minimum UTS),1.1.3 Grade 3—UNS R50550. Unalloyed titanium,1.1.4 Grade 4—UNS R50700. Unalloyed titanium,1.1.5 Grade 5—UNS R56400. Titanium alloy (6 % aluminum, 4 % vanadium),1.1.6 Grade 6—UNS R54520. Titanium alloy (5 % aluminum, 2.5 % tin),1.1.7 Grade 7—UNS R52400. Unalloyed titanium plus 0.12 % to 0.25 % palladium,1.1.7.1 Grade 7H—UNS R52400. Unalloyed titanium plus 0.12 % to 0.25 % palladium (Grade 7 with 58 ksi (400 MPa) minimum UTS),1.1.8 Grade 9—UNS R56320. Titanium alloy (3 % aluminum, 2.5 % vanadium),1.1.9 Grade 11—UNS R52250. Unalloyed titanium plus 0.12 % to 0.25 % palladium,1.1.10 Grade 12—UNS R53400. Titanium alloy (0.3 % molybdenum, 0.8 % nickel),1.1.11 Grade 13—UNS R53413. Titanium alloy (0.5 % nickel, 0.05 % ruthenium),1.1.12 Grade 14—UNS R53414. Titanium alloy (0.5 % nickel, 0.05 % ruthenium),1.1.13 Grade 15—UNS R53415. Titanium alloy (0.5 % nickel, 0.05 % ruthenium),1.1.14 Grade 16—UNS R52402. Unalloyed titanium plus 0.04 % to 0.08 % palladium,1.1.14.1 Grade 16H—UNS R52402. Unalloyed titanium plus 0.04 % to 0.08 % palladium (Grade 16 with 58 ksi (400 MPa) minimum UTS),1.1.15 Grade 17—UNS R52252. Unalloyed titanium plus 0.04 % to 0.08 % palladium,1.1.16 Grade 18—UNS R56322. Titanium alloy (3 % aluminum, 2.5 % vanadium) plus 0.04 % to 0.08 % palladium,1.1.17 Grade 19—UNS R58640. Titanium alloy (3 % aluminum, 8 % vanadium, 6 % chromium, 4 % zirconium, 4 % molybdenum),1.1.18 Grade 20—UNS R58645. Titanium alloy (3 % aluminum, 8 % vanadium, 6 % chromium, 4 % zirconium, 4 % molybdenum) plus 0.04 % to 0.08 % palladium,1.1.19 Grade 21—UNS R58210. Titanium alloy (15 % molybdenum, 3 % aluminum, 2.7 % niobium, 0.25 % silicon),1.1.20 Grade 23—UNS R56407. Titanium alloy (6 % aluminum, 4 % vanadium with extra low interstitial elements, ELI),1.1.21 Grade 24—UNS R56405. Titanium alloy (6 % aluminum, 4 % vanadium) plus 0.04 % to 0.08 % palladium,1.1.22 Grade 25—UNS R56403. Titanium alloy (6 % aluminum, 4 % vanadium) plus 0.3 % to 0.8 % nickel and 0.04 % to 0.08 % palladium,1.1.23 Grade 26—UNS R52404. Unalloyed titanium plus 0.08 % to 0.14 % ruthenium,1.1.23.1 Grade 26H—UNS R52404. Unalloyed titanium plus 0.08 % to 0.14 % ruthenium (Grade 26 with 58 ksi (400 MPa) minimum UTS),1.1.24 Grade 27—UNS R52254. Unalloyed titanium plus 0.08 % to 0.14 % ruthenium,1.1.25 Grade 28—UNS R56323. Titanium alloy (3 % aluminum, 2.5 % vanadium) plus 0.08 % to 0.14 % ruthenium,1.1.26 Grade 29—UNS R56404. Titanium alloy (6 % aluminum, 4 % vanadium with extra low interstitial elements, ELI) plus 0.08 % to 0.14 % ruthenium,1.1.27 Grade 32—UNS R55111. Titanium alloy (5 % aluminum, 1 % tin, 1 % vanadium, 1 % zirconium, 0.8 % molybdenum),1.1.28 Grade 33—UNS R53442. Titanium alloy (0.4 % nickel, 0.015 % palladium, 0.025 % ruthenium, 0.15 % chromium),1.1.29 Grade 34—UNS R53445. Titanium alloy (0.4 % nickel, 0.015 % palladium, 0.025 % ruthenium, 0.15 % chromium),1.1.30 Grade 35—UNS R56340. Titanium alloy (4.5 % aluminum, 2 % molybdenum, 1.6 % vanadium, 0.5 % iron, 0.3 % silicon),1.1.31 Grade 36—UNS R58450. Titanium alloy (45 % niobium),1.1.32 Grade 37—UNS R52815. Titanium alloy (1.5 % aluminum),1.1.33 Grade 38—UNS R54250. Titanium alloy (4 % aluminum, 2.5 % vanadium, 1.5 % iron), and1.1.34 Grade 39—UNS R53390. Titanium alloy (0.25 % iron, 0.4 % silicon).NOTE 1: H grade material is identical to the corresponding numeric grade (that is, Grade 2H = Grade 2) except for the higher guaranteed minimum UTS, and may always be certified as meeting the requirements of its corresponding numeric grade.1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 590元 加购物车

定价： 646元 加购物车

定价： 481元 / 折扣价： 409 元 加购物车

定价： 646元 加购物车

This specification covers rolled nickel-chromium-molybdenum-columbium alloy (UNS No. N06625) and nickel-chromium-molybdenum-silicon alloy (UNS No. N06219) plates, sheets, and strips. N06625 alloys shall be furnished in two grades of different heat treatment conditions, namely: Grade 1 (annealed); and Grade 2 (solution annealed). N06219 alloys, on the other hand, shall be furnished in solution annealed condition only. Materials shall be sampled prepared, and tested accordingly to examine their conformance to dimensional (thickness, weight, width, diameter, length, straightness, edges, squareness, and flatness), mechanical (tensile and yield strengths, and elongation), and chemical composition requirements.1.1 This specification2 covers rolled nickel-chromium-molybdenum-columbium alloy (UNS N06625) and nickel-chromium-molybdenum-silicon alloy (UNS N06219)3 plate, sheet, and strip.1.1.1 Alloy UNS N06625 products are furnished in two grades of different heat-treated conditions:1.1.1.1 Grade 1 (Annealed)—Material is normally employed in service temperatures up to 1100°F (593°C).1.1.1.2 Grade 2 (Solution Annealed)—Material is normally employed in service temperatures above 1100°F (593°C) when resistance to creep and rupture is required.NOTE 1: Hot-working or reannealing may change properties significantly, depending on working history and temperatures.1.1.2 Alloy UNS N06219 is supplied in solution annealed condition only.1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to become familiar with all hazards including those identified in the appropriate Material Safety Data Sheet (MSDS) for this product/material as provided by the manufacturer, to establish appropriate safety, health, and environmental practices, and determine the applicability of regulatory limitations prior to use.1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 590元 加购物车

This specification establishes the requirements for plates, sheets, strips, and rolled bars of copper-cobalt-beryllium and copper-nickel-beryllium alloys with Copper Alloy UNS Nos. C17500 and C17510. The material for manufacture shall be cast and worked to such purity and soundness as to be suitable for processing by hot working, cold working, and thermal treatment to produce a uniform wrought structure and the specified temper. Products shall be produced in tempers TB00 (solution heat treated), or TD02 and TD04 (cold worked), TF00 or TH02 and TH04 (precipitation heat treated), or TM02 and TM04 (mill hardened). Products shall be sampled and prepared, then tested accordingly to examine their conformance to dimensional (thickness, width, length, straightness, and edges), mechanical (tensile strength, Rockwell hardness, and elongation), electrical conductivity, and chemical composition requirements.1.1 This specification establishes the requirements for plate, sheet, strip, and rolled bar. The following alloys are included:2  Previously Nominal Composition, %Copper Alloy Used        UNS No. Designation Beryllium Cobalt Nickel Iron           C17500 Alloy 10 0.6 2.5 ... ...C17510 Alloy 3 or 14 0.4 ... 1.8 ...C71700 ... 0.5 ... 31.0 0.701.2 Units—The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.1.3 The following safety hazard caveat pertains only to the test methods described in this specification:1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

定价： 590元 加购物车