ACERO 2550

TECHNIQUES AND TOOLS FOR QUALITY PRODUCT DESIGN

Product design

There are numerous studies on qualitative approaches in the product design.

Common description of new product development’ is “the process that transforms technical

ideas or market needs and opportunities into a new product on to the market“.

Product development and management Association /PDMA/ defines ‘new product

development’ as “the overall process of strategy, organization, concept generation, product and

marketing plan creation and evaluation, and commercialisation of a new product. [15]

Walsh et al. (1992) describes ‘product design and development’ as “the activity that transforms

the brief or initial market specification into design concepts and prototypes and then into the detailed

drawings, technical specifications and other instructions needed to actually manufacture a new

product. [22]

Business dictionary defines product-design as the detailed specification of manufactured items

parts and their relationships to the whole. A product design needs to take into account how the item

will perform its intended functionality in an efficient, safe and reliable manner. The product also needs

to be capable of being made economically and to be attractive to targeted consumers. [14]

For product design there are several known models based on the specification phase of the

development process.

Fig. 1.

The Design Quality 

Roth [18] definition of design quality is ‘the processes and activities that need to be carried out 

to enable the manufacture of a product that fully meets customer requirements.’ 

Business dictionary defines quality of design as level of effectiveness of the design function in 

determining a product's operational requirements (and their incorporation into design requirements) 

that can be converted into a finished product in a production process. [16] 

Guide Design for Quality definition is - DFQ is the disciplined application of engineering tools 

and concepts with the goal of achieving robust design development and definition in the Pd process. 

The DFQ process allows the engineer to: identify, plan-for and manage factors that impact system 

robustness and reliability upfront in the design process. [1] 

Design quality measures vary from organization to organization. They typically involve some 

measure that attempts to quantify how well the design function achieved certain objectives. These 

objectives can be product specific or they can be aligned with organizational goals. Examples [4]: 

• Carryover parts usage (%), 

• Number of variations for similar products (part count), 

• Change Management, 

• Cost avoidance and cost savings, 

• Product Improvement (number of improvements), 

• Number of technical changes to the product before and after the start of production. 

Design Process Efficiency. Design process measures are metrics intended to quantify the 

efficiency or cost effectiveness of the design process over all engineering design activities. These are 

generally referred to as productivity or efficiency measures. The ones quoted by the panel are: 

• Productivity = (Sales – Materials)/Engineering Labour, 

• Productivity = Engineering cost / Sales (inverse of above), 

• Productivity = (number of part numbers going through the PDP process)/(current year 

engineering expense), 

• % Change in productivity, 

• Project throughput. 

The lean product design 

The essence of Lean is to eliminate waste in all aspects of product development and related 

processes even before getting the product into production. The term is derived from lean 

manufacturing. The starting point is the customer's requirements and determine the value added. All 

others need not satisfy the customer and the customer must pay for it is considered waste. This 

includes: identification of features of the product with the highest added value, delete items without 

value and engage customers in product development stages [19]: 

Vydavatel: Katedra kontroly a řízení jakosti, FMMI, VŠB-TU Ostrava 

• Focusing on the initial development phase, which takes into account many variations, as there is 

room for optimisation. 

• Parallel implementation of activities supported by the communication strategy. 

• Optimise the development process and eliminating waste. 

• Linking specialists from functional departments in multi professional teams. 

• Waste reduction options in the draft. 

. Examples of waste in product design: 

• The proposal was never used, completed or delivered. 

• Downtime in search of information, waiting for test results, etc. 

• Unnecessary documents and prototypes. 

• Insufficient use of product design techniques. 

• Lack of risk analysis of manufacturing defects. 

The best way to eliminate the losses that don’t add value within the process of product design 

and development is to apply the “lean thinking” philosophy. Since “lean” business cannot produce 

“bold” products, the Lean Design and Lean Product Development methods get into concern. Chances 

to dramatic reductions of costs during the product design are: 

• Reduction of direct material costs: platform components and material, simplifying of design, 

reduction of useless waste, samples, prototypes, etc. 

• Reduction of direct costs on experiments and testing simplifying of design - design for lean 

manufacturing and assembly, reduction of part count, adaptation of product tolerances to 

operational possibilities process standardizing, etc. 

• Reduction of operational costs: minimum impact on reconfiguration of manufacturing processes 

and systems, modular design, standards for modifications according to customer’s demands, 

better utilization of manufacturing capacities and human resources. 

• Minimizing development costs: platform of design strategies, lean QFD, Six Sigma, design of 

experiments, value engineering, and others. 

• Acceleration of product development process affects three basic lean principles: 

• Concentration of development activities: perform the work tasks in the shortest time possible, 

and minimum moving of project documentation between individuals and departments. That can 

be achieved with simultaneous solving and strong IT support. 

• Application of knowledge basis from previous experiences portfolio. It means to make use of 

appropriate expertise, learn more than until now and update the knowledge base with 

development-relevant data from suppliers, competitors, customers, and partners. 

Lean companies [20]: 

• Prevent product failures rather than react to them. 

• Create the culture to design quality and reliability into their products. 

• Use product development teams to ensure that the quality and reliability issues of customers, 

manufacturing, service, and suppliers are properly represented. 

• Open communication channels with customers to obtain timely and detailed product failure 

data. 

• Maintain a well-conceived failure database of product field failure modes supported by failure 

analysis to root cause. 

• Understand in detail the capabilities and limitations of both internal and suppliers' 

manufacturing operations. 

Agile Product Development 

The phrase "Agile Product Development" can be interpreted in two ways, both of which are 

correct and applicable to a wide range of products and industries: 

1. An agile product development process that can rapidly introduce a steady succession of 

incremental product improvements which can be called "new" products — that are really planned 

"variations on a theme," based on common parts and modular product architecture. This capability 

results in ultra-fast time-to- market, much faster than possible with independent products that do not 

benefit from product-family synergies in design and manufacture. 

2. Development of agile products that can be manufactured in the following agile environments: Agile 

Manufacturing, Just-in-Time, Build-to-Order, and Mass Customisation. 

Vydavatel: Katedra kontroly a řízení jakosti, FMMI, VŠB-TU Ostrava 

The scope of agility – definitions and overview 

Emend of speed, flexibility, innovation, quality, proactively and profitability through the integration of 

reconfigurable resources that must be achieved in synergy. Quickly react to change by re-

configuration of products, processes and organization structure. [5], [7]. 

Factories, based on Agile Manufacturing and customisation, are characterized [6] by: future 

production sites for a large variety of sophisticated products are offering flexible, short cycle time and 

variability controlling manufacturing capability. These manufacturing approaches ensure energy-

efficient, reliable and cost effective production as well as production set-up/ramp-up with reduced cost 

and time through lean and simpler ICT. The adaptive (agile) enterprises exploit capabilities to thrive in 

uncertain and unpredictable business environment. Firms are capable of rapidly responding to 

changes in customer demand.

The agile manufacturing system should be able to produce a variety of components at low 

cost and in a short time period. To achieve Agile Manufacturing, company need agile design 

processes. [7]. 

Lean product development techniques, many companies have adopted in recent years, 

minimize waste and boost efficiency, but they also lock in product attributes too early and limit 

innovation. 

Agile product development system is capable of addressing frequent iterations of multiple 

design options early in the process, based on continuous testing and highly sophisticated customer-

driven design changes. This method, which both encourages flexibility and recognizes the 

unpredictability of the early stages of product development, ensures that the latter part of the cycle is 

much less uncertain, enabling companies to bring more popular products to market at lower cost, and 

with fewer delays. [7] 

The goal of agile product development is to achieve rapid and frequent iterations with multiple 

design options up front — driven by continuous testing and granular customer analyses — in order to 

optimise, balance, and prioritise requirements and identify risks earlier. This early stage of the process 

has four primary characteristics: 

1. Rapid and iterative development model Companies generate multiple concepts, and in a period of 

weeks, rather than months, test product prototypes with customers. 

2. Modular architecture. By breaking a product concept into modules, companies can give sub-teams 

the responsibility to work out the best set of solutions for the final design and manufacturing of their 

part of the project, including interfaces, materials, or potential trouble spots. 

3. Early risk identification. As cross-functional teams rapidly iterate and synthesize product ideas and 

concepts, more often than not the deep dive into the design process reveals potential development 

risks. With this knowledge, teams can prioritise potential risks and incorporate risk reduction plans into 

the development slate, while scheduling routine test events to verify that risks have been addressed. 

4. Intensive supplier involvement. Traditionally, companies hold suppliers and the manufacturing 

function at arm’s length until product requirements and concepts have matured. By contrast, the agile 

front-end approach seeks to gain the input of all - customers, partners, suppliers, and sales and 

manufacturing teams - to critique designs, offer insights, and broadly minimize risk and maximize 

efficiency up front so that fewer changes need to be made during production or product launch. 

Methods and tools for product innovation 

There is a large collection of techniques, methods and tools to support all phases of product 

innovations. Applications depend on innovation and building innovation potential of the body. Here are 

the comparison of the preferred methods and tools for product design in engineering with an emphasis 

on automobile production [2], [10], [12]. 

Selection tools for conceptual design focuses on the management of: 

• Changing customer preferences, 

• Incorrect specification of the parameters of the products, 

• Different levels of technology maturity, 

• Markets, financial uncertainty and the changing legal, political and social environment. 

Preferred methods are particularly lean and system engineering. A comprehensive set of 

techniques for improving product development can be found in specialist publications. Sample 

selection techniques is in the Table 2. [12]

Other approaches to the selection of techniques for product design: 

Software Design Tools: CAD - Computer Aided Design, CAM - Computer Aided Manufacturing, EDA - 

Electronic Design Automation, DFM - Design For Manufacture, DFT - Design For Test, DFA - Design 

For Assembly. [17] 

Product Design Tools. These would be used predominantly during the design functions, to ensure that 

the right product is specified and designed and to reduce design time and costs. Within the process of 

product design, are there used the tools as: Design for manufacturing/assembly (DFM/A), Design for 

quality (DFQ), Design for Six Sigma (DFSS), Design to cost (DTC), Quality function deployment 

(QFD), Design failure mode and effect analysis (DFMEA). [18] 

In the automotive industry, are widely used methodologies of product quality planning (APQP, VDA 

4.3) and the requirements for the approval process to mass production of parts (PPAP, PPF). 

Advanced product quality planning (or APQP) is a framework of procedures and techniques 

used to develop products in industry, particularly the automotive industry. The purpose of APQP is to 

produce a product quality plan, which will support development of a product that will satisfy the 

customer. APQP serves as a guide in the development process and also a standard way to share 

results between suppliers and automotive companies. 

Part of the quality control standards in the automotive industry is methodological guide PPAP 

(Production Part Approval Process) and PPF (Produktionsprozess-und Produktfreigabe) - Unlocking 

the production process and product. These guides provide a set of requirements for the release of the 

production process and product to manufacture. Their purpose is to determine whether the supplier 

properly understand all the customer's requirements and specifications whether the manufacturing 

process has the potential to produce a product that the requirements will be in the actual production 

volume and at the agreed production speed consistently met. 

The significance of design methods and techniques. In study [3] was made a survey of selected 

frequency techniques in practice, the automotive industry. The survey results reflected in the 0-3 point 

scale are follows: 

• Design for Manufacture and Assembly 2,4 

• Design for Reliability and Durability 2,2 

• Design for Six Sigma 1,0 

• Value Analysis 0,8 

• Design for Service, Repair and Maintenance 0,5 

• Design for Green Manufacturing 0,5 

Shown differentiated approach to product design tool is caused by authors analyze the various 

phases of design and various product sectors. The application design tools are needed specialized 

methodology. In the next part will focus on the results of our research on product design tools to create 

prototypes of cars. 

Case Study: Techniques for Product Design in project of student car 

Testing the potential application of techniques for product design was the Faculty of 

Mechanical Engineering TU Kosice done on a project of student car conducted by the author [9]. The 

project was to design and construct a fully functional car in real size, which represented the University 

faculty and students study the production of cars in their acquired skills and knowledge of the issue of 

product innovations. 

Vydavatel: Katedra kontroly a řízení jakosti, FMMI, VŠB-TU Ostrava 

The car was taken by its originality, aggressiveness, speed, and innovative features modern 

and sporty appearance. The project modelled the real process automakers, with the real constraints 

and didactic intention maximum creative team-based methods learning by doing. 

The sequence of phases of the project were as follows: generation of policy options the car, 

design proposals, graphic and computer design, evaluation, modelling, technical solutions, 

calculations, experiments, production decisions - subcontracting, original equipment manufacturing, 

assembly and testing. 

The new car was built on the Skoda Fabia platform. Body and interior are completely original 

components. The new solution is a hinged door opening. Other innovations are the engine and 

chassis modifications. The new car is shown in Fig. 2.

Project management was a crosscutting activity, as part of the student team was not experience of 

dealing with large and complex projects. 

Lean design methods, because in conditions of university research projects are limited financial and 

technological resources. 

Design for quality, because the car represents the quality of student education in innovation and 

product design techniques. It shows the conditions for their application in automotive research and 

development.

Study of a Steel’s Energy Absorption System for Heavy Quadricycles and Nonlinear Explicit Dynamic Analysis of its Behavior under Impact by FEM

Longitudinal Energy Absorption in Conventional Vehicles
 Passive safety comes into play once an accident is inevitable and its function is to reduce as far as possible the fatal consequences of an impact. On impact, the structure of a vehicle has two main functions: ‚
- The first function is to absorb the kinetic energy of the vehicle while keeping those allowable decelerations for the survival of occupants. ‚
- The second function is to preserve the integrity of the passenger compartment and so avoid the intrusion of rigid components into it.

Energy Absortion Systems

 Studying the structural behavior of a vehicle in the event of longitudinal impact reveals that the structure of a vehicle is composed of components whose function is to deform in a programmed way under impact (deformable parts), i.e., to absorb energy. These components in first instance are the vehicle bumper, and the initial section of the front rails of the vehicle (see Figure 1) [5]. In most conventional vehicles the front rail is made up of two parts: a replaceable piece called the crash box (A), which folds in a controlled pattern under impact, and a second part, the frame rail, with an initial section (B) which also folds in a controlled pattern and a second section (C) which transmits the force to the vehicle structure.

The ideal behavior of the front structure of a vehicle would provide a constant resistance to deformation (while not reaching the passenger safety cell). This would cause a constant force producing deceleration that is not only great but which must also be bearable by the occupants. Various studies focus on the behavior of the axial crushing of tubes [6,7], where folding patterns of different geometries are studied, and the evolution of compressive force versus displacement is tested. The study of geometry buckling initiators [8] is also very common. These are small predeformations in the tube that will achieve a reduction in the initial peak force for the collapse of the tube. The use of buckling initiators is very common in crash boxes and in the initial section of frame rails in order to reduce the initial buckling force.

Energy Absorption Distribution between the Different Components of the Front Structure of an Automobile.

The distribution of energy absorption between different longitudinal components can be studied. Several works have studied this, particularly doctoral theses [9–11], which make a proposal for the division of energy for an impact at 56 km/h against a rigid wall as shown in Figure.

Figure 2 shows a top view of the front of a car, where the distribution of energy absorption can be seen. The front panel of the vehicle absorbs only 10% of the total energy, and the components 6895 Materials 2015, 8, 6893–6908 which absorb more energy are the front rail of the car with 50% of the energy, followed by the motor, which absorbs 20%. Within each of the two front rail components, 7.5% of the total energy is absorbed by the crash box (Figure 1A), 7.5% by the front section of the frame rail (Figure 1B) and another 10% by the frame rail (Figure 1C). These energy absorption percentages would be different depending on the impact speed. If the velocity is not high, the components which are in the first half of the structure would be able to absorb all the kinetic energy, without deformation of the second half. These percentages correspond to an impact that does not involve deformation of the passenger compartment and the impacted object is a rigid wall which does not absorb energy.

Impact Performance of Minicars

Now that the impact performance for conventional vehicles has been shown, we will focus on the performance for quadricycles and minicars. The literature on the impact crashworthiness of minicars has been mostly written relatively recently and in all events is very scarce. One of the most complete references is the study by Hardy [12], a European study of L category vehicles (unladen mass under 350 kg or 450 kg) and whether they could meet the same regulatory requirements as M1 cars (conventional cars). The study shows wide disparity between the results of frontal-impact tests for the quadricycles reviewed, and indicates that no current quadricycle would comply with M1 category safety requirements, which suggests that the evolution of safety in quadricycles still has a long way to go. Moreover, EuroNCAP (European New Car Assessment Programme) undertook a safety campaign for heavy quadricycles (L7e) [13] in 2014, which tested the following models: Renault Twizy 80, Ligier IXO JS Line 4 Places, Tazzari Zero and Club Car Villager 2+2 LSV. The study results indicate that “all of the quadricycles tested showed critical safety problems”, and according to the executive management of ETSC (European Transport Safety Council) “these vehicles already satisfy a minimum set of requirements which is clearly not enough as the tests show” [13]. Mizuno [2] produced a work on passive safety for minicars, which clearly states that their security is less than that of a conventional vehicle due to the technological challenge of their small size and mass. The work concludes that the lack of safety is mainly due to the lack of space, and that a lower mass implies larger decelerations, and it recommends that this type of vehicle have a force limiter and pre-tensioner system for the seat belt and shrinkable columns for the steering because intrusions into the passenger compartment are one of the main problems with this type of vehicle. In a way, the chassis structure of a minicar is similar to that of a conventional vehicle, with the handicap of having less space available. Figure 3 shows the chassis structure of two minicars whose impact performance was tested [5]. Minicar type A (see Figure 3a) has two front rails connected directly to the crossbeam that supports the bumper, while minicar type B (see Figure 3b) does not have a crossbeam for the bumper and front rails are joined together by a horizontal beam over the suspension. Materials 2015, 8, page–page 4 motor, which absorbs 20%. Within each of the two front rail components, 7.5% of the total energy is absorbed by the crash box (Figure 1A), 7.5% by the front section of the frame rail (Figure 1B) and another 10% by the frame rail (Figure 1C). These energy absorption percentages would be different.

JOMINY TEST APPLIED TO PM STEELS FOR HEAT TREATMENT

The hardenability of steels, as defined by ASM,1,2 is usually measured as the distance below a quenched surface at which the metal shows a specific hardness or a specific percentage of martensite in the microstructure. To rate the hardenability, two methods can be used: the Grossman test and the Jominy test. The latter3–5 is the simpler and the more commonly used one. While for fully dense steels the literature data cover a wide variety of grades, for PM steels the available database seems to be mainly limited to rather recent years.6–12 As everybody knows, the hardenability of PM steels depends on:
(i) chemical composition, that can be uneven, depending on powder nature and processing conditions (ii) austenitic grain size mainly depending on sintering condition
(iii) cooling rate inside parts within a given range. This rate depends on material (chemical compositions, density and sintering intensity) and on part geometry and size.

The main differences between PM and fully dense materials are the presence of diffused porosity and, in many cases, the typical heterogeneity of chemical compositions.12–16 Owing to the growing interest in sinter hardening, the knowledge of hardenability of PM steels is becoming more and more important to establish the maximum part weight and typical geometrical features of parts suitable for sinter hardening. In some previous papers,17–20 the hardenabilities of PM steels – based on some ferrous base powders – have been determined, through the Jominy end quench test and hardness measurements, as suggested by the standard. Also the possibility of obtaining ferrous materials characterised by hardness values (200–250 HV) suitable for specific mechanical applications by means of a sinter–hardening process applied to low or medium alloyed steels has been investigated.13 In this paper the Jominy test has been applied to PM ferrous materials produced from low or medium alloyed powders, suitable for quenching heat treatment in the case of demanding technical requirements.

Materials and methods Two low alloy powders, produced by Ho¨ gana¨ s AB (Sweden) have been considered; their chemical compositions are reported in Table 1. These grades, produced also by other manufacturers, are frequently selected for producing PM parts requiring heat treatment. Prisms with square cross-section (65665 mm2 ) and 63 mm height have been compacted at 7. 0 g cm23 nominal density and sintered under endogas atmosphere at 1393 K for 30 min. in belt type industrial equipment. After sintering and physical characterisation, each prism has been cut into four equal parts and from each quarter, by turning, a cylinder has been machined. The dimensions of the cylinders are: 25 mm in diameter and 63 mm high. Then, on each cylinder, at the centre, a threaded hole has been machined. This hole has been made to allow the insertion of a threaded extremity part,

1. Hardenability bands of investigated steels

in order to observe the requirement of the standard, which imposes a total length equal to 100 mm.4,5 Other three holes, equally spaced, have been machined on the specimens to make possible the insertion of thermocouples, with extremities placed at different, precise distances from the quenched end. The instrumented Jominy probes have been placed inside a muffle furnace, set at 900uC, with an hour time at temperature. Since no controlled atmosphere was available, to avoid carbon depletion on the surface zones, the specimens were placed inside small cylinders equipped with a graphite disc at the bottom. In order to follow the temperature course, three thermocouples have been inserted inside the Jominy probes, placed at half of the base radius and at distances of 3, 13 and 23 mm from the quenched surface. During the cooling after austenitising, at the three selected points, the temperatures have been recorded through a digital acquisition system. The present investigation includes: 

(i) measurement of Rockwell A hardness 

(ii) measurement of Vickers 0. 05 microhardness 

(iii) optical (LOM) and electronic (SEM) microscopy (iv) EDXS analysis.

Experimental results
Hardenability bands and microhardness 

After cooling in the Jominy test device, each probe has been ground along four generatrices, arranged at 90u each other, on which hardness measurements have been made. From the hardness measurements (Rockwell A) Jominy curves have been obtained for both materials. The results of these measurements are reported in Fig. 1. Along one generatrix, selected at random, also the HV0. 05 microhardness profile has been determined, with 1 mm pitch. The results are reported in Fig. 2, where each point of the profile is the average value of three measurements at that distance

The hardenability bands, depicted in Fig. 1, indicate some difference in hardenability between the investigated steels. Anyhow, assuming 70 HRA as the lower limit for a ‘hard’ structure, for both materials the ‘design’ hardening depth is y5 mm. Figure 2, for the steel obtained from AE base powder, shows that proceeding from the quenched face, on the first 4 mm, microhardness profile exhibits high values (800–860 HV0. 05), then the microhardness decreases very quickly, dropping to 400 HV0. 05 in the next 3 mm. From this point on, the profile continues to drop, but softly and slowly, till 350 HV0. 05. On the same figure, the hardness profile of the material obtained from Astaloy Mo shows microhardness values between 760 and 780 in the first eight measurements (always starting from the quenched end). Then, in the distance interval from 8 to 20 mm, the microhardness values are nearly halved, dropping to y400 HV0. 05. After this point, the microhardness decreases slowly, to reach 320 HV0. 05 in the last measurements. The different statistical populations of microhardness values are shown in Figs. 3 and 4, on normal probability graphs. Each plot represents the results of 450 measurements. Figure 3, relevant to AE coded PM steel, shows the overlapping of at least four different statistical populations. Less than 10% of the values exceed.

650 HV0. 05; they are concentrated within ,15 mm from the water quenched end. The median is y350 HV0. 05. Figure 4, relevant to AM coded PM steel, shows the overlapping of at least three statistical different populations. About 25% of the values exceed 650 HV0. 05; they are concentrated within ,30 mm from the water quenched end. The median is y370 HV0. 05. 

Microstructures 

Typical microstructures have been observed at increasing distances from the water quenched end of each Jominy specimen. No significant differences have been observed among probes of the same chemical family.

AE coded material 

The extreme part of the probe nearly completely presents a martensitic structure with some zones of transforming austenite (Figs. 5 and 6). The white areas observed at the optical microscope immediately below the quenched face are seen to be completely transformed when observed with the SEM (Fig. 7). Almost immediately, beyond this area, the microstructure suddenly changes and appears to be made up of very fine pearlite and austenite transforming in acicular structures (Figs. 8 and 9). Then, going on along the sample, very fine pearlite predominates with small quantity of upper bainite and austenite at different degrees of transformation (Figs. 10–13).

Then, at .5 mm distance from the quenched end, lower bainite becomes the predominant structure (Fig. 17). At y18 mm from the quenched face the presence of upper bainite becomes remarkable, intermixed with only some areas of lower bainite (Figs. 18–20). The areas white at LOM (Fig. 18), when observed at SEM, appear completely transformed in upper bainite (Fig. 21). Furthermore, at 20 mm distance, the structure is prevalently constituted by upper bainite, with the presence of pearlitic structures (Figs. 22–24). For the steels based on diffusion bonded powders (materials coded AE), EDXS analysis pointed out

uneven compositions as expected. The range of local Ni amount (wt-%) has been between 2 and 8% as usual at 1393 K sintering temperature with industrially acceptable times. Concluding remarks The results of this investigation confirm the good hardenability of both PM steels, but for limited distances from the quenched end. The hardenability bands indicate that the typical hardening depth is y5 mm for both materials, After this distance, the change towards non-martensitic structures is sudden for the steel from diffusion bonded powder and more soft for the steel based on Mo prealloyed powder. It should be observed that the hardenability bands are based on

HRA hardness instead of HRC hardness, as prescribed by the standard and customarily used for fully dense steels. At first sight, the use of a different scale could be criticised. It can mostly be observed, however, that when reducing the test load from 150 to 60 kp the probability of overtaking the border of reliable hardness measurements notably decreases. As we know, any Rockwell value is obtained by the difference between two positions of the indenter. The lower the difference between position, the higher the error of measurement. For this reason, any Rockwell value below 30/35 is affected by a measurement error that can alter the ‘true’ properties of the investigated materials, independently from any other variable. According to a well consolidated experience, 70 HRA corresponds to 39. 25 HRC.21 If the weakening effect of y11% porosity is duly considered, it should be unquestionable that 39 HRC on such porous steel can be achieved only if the microstructure is fully martensitic. So, the choice of the hardenability limit is based on these considerations. The data obtained from the experiments show that:

(i) as expected, the Jominy test is apt to rate the hardenability of PM steels

(ii) the hardenability bands and the microhardness profiles agree 

(iii) the microstructure features and the microhardness distributions agree 

(iv) the AE coded material shows high microhardness values (800–860 HV0. 05) only on the first 4 mm from the water quenched end. From this distance on, the microhardness decreases very quickly to 400 HV0. 05 at 7 mm distance. After this point the microhardness profile presents a slow drop to y300 HV0. 05 at .50 mm distance, i.e. at the centre of the standard Jominy specimen 

(v) near the water quenched end the AM coded material reaches lower microhardness values (760–780 HV0. 05) than the AE coded material, considering the same zone. However these values remain constant at y8 mm. Beyond this point the microhardness begins to decrease slowly, reaching 400 HV0. 05 at 20 mm from the water quenched end 

(vi) the AE coded material, at .5 mm distance from the quenched end, presents some austenite areas surrounding the pores. These areas are typically Ni rich, so that they prevent or obstruct the formation of martensite. The location of these

areas depends on the powder nature, where the diffusion–bonding process fixed very fine nickel particles on the surface of coarser iron at random. During sintering, the nickel facing the pores after compaction can diffuse only in one direction, while, again during sintering, the nickel situated between contacting particles after compaction can diffuse in two directions. In the first case, the concentration gradient will be inevitably more pronounced than in the second case. It is well known that the Ni rich austenite areas are definitely positive in the case of fatigue stresses acting in operation. On the contrary, their hardness and the corresponding wear resistance are rather poor. This latter remark enables to propose a distinction between the two materials. The distinction is based on the difference between the pattern of applied load. At equal hardness, in the case of prevailing and continuous wear stress, the PM steels based on diffusion bonded materials can be less resistant than the PM steels based on prealloyed powders. The contrary should be the operating behaviour in the case of combined stresses, including fatigue. This difference agrees with the common selection criteria followed by PM part manufacturers and users

TALLER DE DIFUSION NELSON Y FELIPE RAMIREZ

11)  Una oblea de silicio de 0.3mm de espesor es tratada de manera que se produzca un gradiente de concentración uniforme de antimonio. Una de las superficies contiene 1 átomo de Sb por cada 10 átomos de Si, el parámetro de red del Si es de 5,4307ª. Calcules el gradiente en:

      A) porcentaje atómico de Sb por centímetro
B) átomos  de Sb/cm3

2

2)   Cuando una aleación de CuZn se edifica, una parte de la estructura contiene 25% atómico de Zinc y otra porción a 0.025mm de distancia contiene 20% de Zinc, el parámetro de la red para la aleación FCC es aproximadamente 3,63X10-8cm. Determine el equivalente de concentración en:
A) porcentaje de Zn por cm
B) porcentaje en peso por Zn por cm
C) átomos de Zn

3

3)  Determine la temperatura máxima permisible que produzca un flujo de menos de 2000 átomos H/cm2 a través de una hoja de hierro BCC, cuando el gradiente de concentración es de -5X1016 átomos/cm2

4

4)  Compare el coeficiente de difusión para el hidrogeno y el nitrógeno en hierro FCC a 1000C y explique la diferencia.

5

5)  Se realiza un proceso de carbonización en un acero con 0,10% C, introduciendo 1,0% en la superficie a 980C, temperatura a la cual el hierro es FCC. Calcule el contenido de carbono a 0,01cm, 0,05cm y 0,10cm por debajo de la superficie después de haber pasado una hora.

  

                                                        Desarrollo

Ya hallando D se reemplaza en la primera ecuación para encontrar el tiempo.

4.   

Hidrogeno

      Nitrógeno

El coeficiente de difusión del nitrógeno es menor que el del hidrogeno por lo cual el hierro actuando a 1000C, el hierro se difunde más en material.

5.     

Como el porcentaje de Cx es el que nos falta se despeja de la anterior ecuación.

Para X=0,05

Para X=0,10