鍛件熱處理淬火時(shí)的缺陷及預(yù)防措施

1)氧化與脫碳

鍛圓廠家的鋼在加熱時(shí)，表面有一層松脆的氧化鐵皮的征象稱為氧化；脫碳指表面含碳量低落的征象。氧化和脫碳會低落鍛件上層的硬度和委靡強(qiáng)度，并且還影響鍛件的尺寸。為了防止氧化和脫碳，通常在鹽浴爐內(nèi)加熱，要求更高時(shí)，可在鍛件表面涂覆保護(hù)劑或在保護(hù)空氣及真空中加熱。

2)過熱和過燒

鍛件在淬火加熱時(shí).奧氏體晶粒顯著粗化的征象稱為過熱。若加熱溫度過高，出現(xiàn)晶界氧化并首先片面融化的征象稱為過燒。鍛件過熱，不僅會低落鋼的力學(xué)機(jī)能（尤其是韌性），也容易惹起淬火變形和開裂。過熱構(gòu)造可以用正火處分予以改正，而過燒的鍛件只能報(bào)廢。為了防止鍛件的過熱和過燒，必須嚴(yán)酷控制加熱溫度和保溫光陰。

3)變形與開裂

鍛件淬火冷卻時(shí)，由于差別部位存在溫度差異及構(gòu)造轉(zhuǎn)變的差別所惹起的應(yīng)力稱為淬火內(nèi)應(yīng)力。當(dāng)淬火應(yīng)力超過鋼的降服強(qiáng)度時(shí)，鍛件將產(chǎn)生變形；當(dāng)淬火應(yīng)力超過鋼的抗拉強(qiáng)度時(shí)，鍛件將產(chǎn)生裂紋而成為垃圾。為了防止鍛件的變形和開裂的產(chǎn)生，可采用差別的淬火技巧 (如分級淬火或等溫淬火等）和工藝合理的計(jì)劃措施（如布局對稱、斜面勻稱、幸免尖角等），盡管削減淬火應(yīng)力，并在淬火后及時(shí)舉行回火處分。

4)硬度不及

由于加熱溫度過低、保溫光陰不及、冷卻速度不敷大或表面脫碳等緣故變成硬度不及，可采用從新淬火的技巧來消除（但淬火前要舉行一次退火或正火處分），并且還須留意以下兩點(diǎn)差別。

(1)淬透性與實(shí)際鍛件的有效淬硬深度的差別。采用同一種鋼、差別截面的鍛件在一樣奧氏體化前提下淬火，其淬透性是相像的，不過其有效淬硬深度卻因鍛件的樣式、尺寸和冷卻介質(zhì)的差別而有差別。淬透性乃是鋼本身所固有的特征，關(guān)于一種鋼，它是確定的，可用于差別鋼種之間的對照。而實(shí)際工件的有效淬硬深度，它除了取決于鋼的淬透性外，還與鍛件的樣式、尺寸及采用的冷卻介質(zhì)等外界成分相關(guān)。

(2)鋼的淬透性與淬硬性是兩個(gè)差別的觀點(diǎn)，淬硬性是指鋼淬火后能到達(dá)的非常高硬度，它要緊取決于馬氏體的含碳量。淬透性好的鋼其淬硬性不必然高。例如低碳合金鋼淬透性相當(dāng)好，但其淬硬性卻不高；高碳鋼的淬硬性高，但其淬透性卻差。

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Defects and preventive measures for heat treatment and quenching of forgings

1) Oxidation and decarburization

When the steel of the forged round manufacturer is heated, the surface of the surface has a layer of crunchy iron oxide scale called oxidation; decarburization refers to the sign of low carbon content on the surface. Oxidation and decarburization will lower the hardness and euphemism of the upper layer of the forging and also affect the size of the forging. In order to prevent oxidation and decarburization, it is usually heated in a salt bath furnace. When higher requirements are required, a protective agent may be applied to the surface of the forging or heated in a protective air and a vacuum.

2) Overheating and overburning

When the forging is heated by quenching, the sign of significant coarsening of austenite grains is called overheating. If the heating temperature is too high, the sign of grain boundary oxidation and first melt on one side is called over-burning. Overheating of the forgings not only reduces the mechanical properties of the steel (especially toughness), but also causes quenching deformation and cracking. The overheated structure can be corrected by normalizing the fire, and the overheated forgings can only be scrapped. In order to prevent overheating and overheating of forgings, the heating temperature and the holding time must be strictly controlled.

3) Deformation and cracking

When the forging is quenched and cooled, the stress caused by the difference in temperature and the difference in structural transformation between the different parts is called the internal stress of quenching. When the quenching stress exceeds the falling strength of the steel, the forging will be deformed; when the quenching stress exceeds the tensile strength of the steel, the forging will generate cracks and become garbage. In order to prevent deformation and cracking of forgings, different quenching techniques (such as step quenching or isothermal quenching) and process planning measures (such as layout symmetry, symmetry of the bevel, and sharp corners) can be used, although the quenching stress is reduced. And after the quenching, timely tempering is carried out.

4) less than hardness

Because the heating temperature is too low, the heat preservation time is not enough, the cooling rate is not enough, or the surface decarburization becomes the hardness, it can be eliminated by the new quenching technique (but an annealing or normalizing treatment is performed before quenching), and Be aware of the following two differences.

(1) The difference between the hardenability and the effective hardening depth of the actual forging. The same steel and differential section forgings are quenched under the same austenitizing conditions, and the hardenability is similar, but the effective hardening depth is different due to the difference in the style, size and cooling medium of the forging. Hardenability is an inherent feature of steel itself. For a steel, it is deterministic and can be used for comparison between different steel grades. The effective hardening depth of the actual workpiece, in addition to the hardenability of the steel, is related to the external components such as the style and size of the forging and the cooling medium used.

(2) The hardenability and hardenability of steel are two different viewpoints. Hardenability refers to the very high hardness that steel can reach after quenching, and it depends on the carbon content of martensite. The hardenability of steel with good hardenability is not necessarily high. For example, low carbon alloy steel has a good hardenability, but its hardenability is not high; high carbon steel has high hardenability, but its hardenability is poor.